Blanch et al (2011) - Biomass deconstruction to sugars.pdf

8/11/2019 Blanch et al (2011) - Biomass deconstruction to sugars.pdf

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BiotechnologyJournal

DOI 10.1002/biot.201000180 Biotechnol. J. 2011, 6, 10861102

1086 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

The conversion of the carbohydrate content of bio-mass into transportation fuels relies on formationof sugars by hydrolysis of the glycosidic links pres-ent in the cellulose and hemicellulose componentsof biomass.Enzymatic hydrolysis provides a meansof accomplishing this with near complete conver-sion of both hexose and pentose saccharides tomonomers. In contrast, dilute acid (25%, 160200C,and 10 atm) hydrolysis results in the forma-tion of by-products that reduce yields. Concentrat-ed acid (1030%, 50C, and 1 atm) hydrolysis canprovide near complete conversion of cellulose to

glucose, but the cost of the acid can be significant.The use of cellulase enzymes for hydrolysis undermild conditions (50C, pH 5) is thus an attractiveapproach, provided the costs of the enzymes andthe reaction time required can both be reduced.Ac-

complishing this relies on the pretreatment of bio-mass to provide surface area that is accessible toenzymatic action, the release of hemicellulose fromthe lignin, and decrystallization of cellulose. Ac-etate is commonly released from hemicelluloseduring pretreatment processes and, together withother compounds that also result from pretreat-ment, are inhibitory to subsequent fermentation ofthe sugar monomers to fuels such as ethanol [1].Reducing their formation is a further objective ofpretreatment processes.

There are several physical,chemical, and physi-cochemical methods that have been developed topretreat biomass prior to enzymatic hydrolysis.

Physical methods include mechanical size reduc-tion (comminution),and less commonly, irradiationof the biomass by -rays [2], the latter being toocostly for commercial consideration. Chemicalmethods include acid or base addition at elevatedtemperatures. Dilute sulfuric acid (typically below4%) is effective in breaking down hemicelluloseand providing improved accessibility for cellulosehydrolysis. Other chemical methods alter the lignincomponent of biomass. Solvents such as ethanol ormethanol, mixed with an aqueous inorganic acidcatalyst such as HCl or H2SO4, are effective in

Review

Biomass deconstruction to sugars

Harvey W. Blanch1,2, Blake A. Simmons1,3 and Daniel Klein-Marcuschamer1

1 Joint BioEnergy Institute, Emeryville, CA, USA2 Lawrence Berkeley National Laboratory, Berkeley, CA, USA3 Sandia National Laboratories, Livermore, CA, USA

The production of biofuels from lignocellulosic biomass relies on the depolymerization of its poly-

saccharide content into fermentable sugars. Accomplishing this requires pretreatment of the bio-

mass to reduce its size, and chemical or physical alteration of the biomass polymers to enhance

the susceptibility of their glycosidic linkages to enzymatic or acid catalyzed cleavage. Well-studied

approaches include dilute and concentrated acid pretreatment and catalysis, and the dissolution

of biomass in organic solvents. These and recently developed approaches, such as solubilization

in ionic liquids, are reviewed in terms of the chemical and physical changes occurring in biomass

pretreatment. As pretreatment represents one of the major costs in converting biomass to fuels,

the factors that contribute to pretreatments costs, and their impact on overall process economics,

are described.

Keywords: Biofuels Lignocellulosic biomass Pretreatment White/Industrial biotechnology

Correspondence: Dr. Harvey W. Blanch, Joint BioEnergy Institute,

5885 Hollis Street, Emeryville, CA 94608, USA

E-mail: [email protected]

Abbreviations: AFEX, ammonia fiber explosion; ARP, ammonia recycled

percolation

Received 17 December 2010

Revised 1 May 2011

Accepted 6 June 2011


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2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1087

Biotechnol. J. 2011, 6, 10861102 www.biotechnology-journal.com

delignifying biomass. The reduction in lignin con-tent by pretreatment results in improved enzymat-ic conversion of cellulose to glucose [3]. Certainionic liquids (ILs) have been recently shown to dis-solve biomass [4], and the addition of an anti-sol-vent such as water enables the cellulosic compo-

nents to be precipitated prior to enzymatic hydrol-ysis [5].

There have been a number of recent reviews ofbiomass pretreatment, with a focus on acid andbase methods [69]. The present review brieflysummarizes these approaches and presents moredetail on recent methods that primarily delignifybiomass, including organosolv and IL pretreat-ments.

2 Pretreatment methods

2.1 Physical methods

2.1.1 Mechanical methodsProviding adequate surface area for enzymatic hy-drolysis is key to reducing hydrolysis times. Sever-al approaches can be employed to initially reducebiomass size, including chipping, milling andgrinding. Biomass is first chopped to an initial sizeprior to further size reduction. Hammer and knifemills have most commonly been employed for fur-ther size reduction in laboratory and small pilotscale research, and at larger scale are used to grindalfalfa chops to produce pellets. The energy re-quirements for comminution depends on the initialsize of the biomass, its moisture content and phys-ical properties, as well as the rate of feeding to themilling equipment. Higher moisture contents re-quire more energy input for comminution. Mani etal. [10] provide a summary of previous studies onbiomass grinding. Chipping results in material1030 mm in size,while grinding and milling resultin small particles (0.22.0 mm).

The power requirements for comminution de-pend on the nature of the biomass feedstock. Pro-vided the final particle size is in a range of 36 mm,

the energy required is typically below 30 kWh/tonne biomass [11]. Switchgrass has a relativelyhigh energy requirement of 27.6 kWh/tonne whenhammer milled; in contrast corn stover requires11.0 kWh/tonne when milled to the same size(3.2 mm) [10]. With a harvest moisture contentof ~15%, switchgrass has an energy content of14.8 MBTU/Mg (4337 kWh/tonne) [12], and thusthis comminution energy requirements representless than 1% of the total switchgrass energy con-tent. However, for municipal solid waste, size re-duction below 75 m requires an energy input

greater than that available in the feed [13]. Forhardwood chips,course size reduction (0.20.6 mm)required 2040 kWh/tonne, whereas for reductionto 0.150.3 mm,100200 kWh/tonne of grinding en-ergy is required [13]. Figure 1 illustrates the rela-tionship between final particle size and power re-quirements for several types of milling.A relation-ship between the rate of enzymatic hydrolysis andthe particle size of Avicel (20100 m), at loadingsup to 2 wt%, indicates the importance of reducingparticle size [14].

2.2 Chemical methods

Chemical methods for biomass pretreatment havetheir origin in commercial pulping processes for pa-per manufacture. These processes were developedto retain the strength and integrity of paper pulp,which has a high value (around $1000 per metric tonfor long-fiber, bleached softwood Kraft pulp). How-ever, the objectives for pulping do not correspond tothose desired for biomass to be used in the produc-tion of biofuels, and hence this approach is not used

for pretreatment. Nevertheless, Kraft pulping,which employs NaOH and sodium sulfide at elevat-ed temperatures and pressures, results in the cleav-age of ether links in lignin by nucleophilic sulfide(S2) or bisulfide (HS) ions,producing a readily en-zymatically hydrolyzable material. Kraft pulpingalso dissolves lignin to a significant extent, and in-creases the accessible surface are of the cellulose.

2.2.1 Concentrated acid hydrolysis of biomassConcentrated hydrochloric and sulfuric acids andanhydrous HCl gas have been used to hydrolyze

Figure 1. Comminution of lignocellulosic biomass with hammer and knife

mills. Data from Mani et al. [10] and Cadoche et al. [11].


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the holocellulose content of biomass. Crystallinecellulose is completely soluble in 72% sulfuric acidand in 42% hydrochloric acid. HCl is able to pene-trate wood and its volatility assists in its recovery.Wood saccharification processes were developed inGermany during WWII. The Bergius-Rheinau

process employed 20% HCl, and produced a 10%sugar solution [15], which could be increased to40% by recycle of the HCl solution. Distillation atreduced pressure at a temperature of 36C recov-ered 80% of the HCl, but this was not sufficientlyhigh for economic viability. The UdicRheinauprocess attempted to overcome this limitation [16].Hemicellulose was first removed by pre-hydrolysiswith 1% HCl at 130C, the wood was then dried andhydrolyzed with 40% HCl at 22C for 10 h. Thelignin residue was removed by washing and theHCl recovered by vacuum distillation. It was claim-ed that HCl losses were reduced to 6%, making theprocess economically attractive. Other approachesusing low and high pressure HCl are summarizedin [17]. Efforts today to employ HCl for hydrolysisrely on HCl recovery using quaternary or tertiaryalkyl amine solutions (e.g., www.hclcleantech.com[18]).

Concentrated sulfuric acid can also be used foralmost complete hydrolysis of wood. It has the ad-vantage that the sugars produced are not decom-posed under the reaction conditions employed.However, recovery of sulfuric acid is particularlydifficult, as both the sugars and the acid are non-volatile.The best known approach is the Hokkaidoprocess, developed by the Hokkaido Forest Re-search Institute in Japan around 1948 [19].Wood ispre-hydrolyzed with 0.25 N sulfuric acid at 140150C, then dried, crushed and contacted with 90%sulfuric acid at room temperature for about 30 s.Af-ter filtration and washing, the sulfuric acid is re-covered (~80%) using a dialysis membrane; about2% of the sugars are lost.About 85% of the availableglucose can be obtained,but the low recovery of thesulfuric acid has not made the process economical-ly attractive.

2.2.2 Dilute acid pretreatment and hydrolysisThe difficulties in recovering concentrated acidsled to the use of dilute acids to pretreat and hy-drolyze biomass.One of the major drawbacks in theuse of dilute acids, however, is need to employ ele-vated temperatures, which leads to the degradationof the sugars to furfurals. Both cellulose and hemi-cellulose are converted to monomers and subse-quently to degradation products by essentiallyfirst-order processes [20]:

Both acid-catalyzed reactions can be written as;k1 and k2 have Arrhenius tem-

perature dependencies and the reactions have firstorder dependencies on the effective acid concen-tration. At high temperatures, H2SO4 dissociates to[HSO4]

and provides only 1 mol of acid equivalent.With a knowledge of activation energies and rateconstants, the optimal time and temperature can befound to maximize production of intermediate B,the desired saccharides [21].There is a wide varia-tion in reported rate constants for xylans,which re-flects primarily the variation in composition andcontent in biomass, the biomass buffering capacity,and to a lesser extent the analytical techniquesused and models employed in interpreting the data[22]. McMillan [20] provides a very useful critiqueof the literature on dilute acid hydrolysis kinetics.Data can be grouped into those obtained for lowsolids loading (510 wt%), continuous processes athigh temperature (T> 160C) with low sulfuric acidconcentrations (0.1%); and those for high solidsloadings (1040 wt%), batch processes at low tem-peratures (T< 160C) with 0.73.0% sulfuric acid.

In a typical process, wood or biomass is groundto a size as small as possible (around 1 mm) in or-der to facilitate the permeation of acid into thewood. Hemicellulose can be completely removedby pre-hydrolyzing woody or herbaceous biomass,for example, 0.05 N H2SO4 at 140C for 3060 min,or 510 min at 160C. For corn stover, an optimumcondition was reported as 2% H2SO4 at 120C for43 min, yielding 77% xylose and 8.4% glucose re-lease; the remaining solid phase was readily enzy-

matically hydrolyzed, providing 42 g glucose/100 gof substrate (70% conversion) [23]. Cellulose hy-drolysis occurs when 0.05 N H2SO4 is added underpressure. While good improvements in enzymatichydrolysis of the cellulose content of acid-pretreat-ed biomass can be obtained, the cost of dilute acidpretreatment is higher than physicochemical pre-treatments, and neutralization of the productstreams for enzymatic hydrolysis (pH ~5.0) and fer-mentation is required. The range of degradationproducts and their inhibitory effects on yeastethanol fermentation has been explored [24].

A B Ck k1 2

hemicellulosepentosehexose

oligo-sacc

kX3

hharides

degradationproducts

furfural

kX4

( ))

cellulose

glucose

cellobiose

oligomers

k kG1 GG2

degradation

products

(hydroxymethyl-furrfural)


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2.2.3 Base pretreatmentIn contrast to dilute acid pretreatment, alkalinepretreatment can be conducted at low tempera-tures with less degradation of sugars, but pretreat-ment times are typically longer (hours to days).Theaddition of alkaline swelling agents to biomass im-

proves the accessibility of the biomass to hydroly-sis. Mild swelling agents such as NaOH, hydrazine,and anhydrous NH3 reduce cellulose crystallinity,disrupting the intracrystalline regions and allow-ing the agents to further penetrate into the crystalstructure.It has been proposed that alkaline agentssaponify the uronic ester linkages of 4-O-methyl-D-glucuronic acids attached to the chain of xylanhemicelluloses [25], producing a charged carboxylgroup and reducing cross-linking to lignin and oth-er hemicelluloses. In the case of aqueous orgaseous ammonia, ammonolysis of the same link-age produces an uncharged amide and conse-quently less swelling than is the case with NaOH.Achange in cellulose crystal structure from cellu-lose-I to cellulose-II or III has been reported [26,27] with alkaline pretreatment.

Delignification or lignin relocalization are fea-tures of dilute alkaline pretreatment.This dependson temperature and the presence of oxygen, andresults from the cleavage of the linkages betweenlignin and hemicellulose. Low temperature NaOHpretreatment avoids formation of some of thebyproducts associated with high temperature alka-line pretreatment, and the addition of urea hasbeen shown to be beneficial in releasing carbohy-drates and dissolving cellulose [28]. A number oftime-temperature profiles have been reported forNaOH and lime pretreatments, with Ca(OH)2 load-ings of 0.10.5g/g biomass [9].A comparison of fourpretreatment methods (dilute acid, lime, aqueousammonia steeping followed by dilute acid, andsodium hydroxide) indicated that 2% NaOH-pre-treated corn stover provided the best removal oflignin and release of sugars following enzymatichydrolysis at 20 FPU/g biomass [3].Typically, expo-sure of biomass to NaOH solutions has been con-ducted at high temperatures (>100C), however it

has been shown that lower temperatures (ambientto 55C) is also effective, but requires longer expo-sure times. For switchgrass, studies of NaOH pre-treatment indicated that exposure to 1.0% NaOH at50C for 12 h resulted in optimal yields of reducingsugars (453 mg/g biomass) [29]. Longer exposuretimes and higher NaOH concentrations increasedthe lignin removal.

2.2.4 Solvent-based methodsOrganosolv. The use of hot ethanol to pretreatwoody biomass has its origins in providing a clean

biomass fuel for turbines and subsequently as analternative to Kraft pulping (developed as theCanadian Alcell process [30]). The organosolvpulping process produces a high-quality ligninproduct that serves as a valuable byproduct. Thisapproach has been modified for biomass pretreat-

ment for biofuels production, where a high-qualitylignin byproduct may not be as important. Inorganosolv pretreatment,an aqueous/solvent mix-ture is heated to 100250C with an inorganic acidcatalyst (typically H2SO4) to break the hemicellu-lose-lignin bonds. Alternatively, organic peracidscan be used at lower temperatures,or organic acidssuch as formic, oxalic, acetylsalicylic, and salicylicacids can be employed. Organic solvents includelow boiling alcohols such as ethanol and methanol,higher boiling solvents (ethylene glycol, glycerol,and tetrahydrofurfuryl alcohol) and a variety ofethers, ketones, and phenols.At temperatures over185C, the addition of mineral acids may not be re-quired, as organic acids are released from the bio-mass. However, added acids improve delignifica-tion of the biomass and solubilization of the hemi-cellulose fraction.Organosolv hydrolyzes the lignininternal linkages (including -aryl ether bonds), aswell as the lignin-hemicellulose bonds (ether and4-O-methylglucuronic acid ester bonds to -C ligninlinkages), in addition to hydrolyzing glycosidichemicellulose linkages (and possibly cellulose un-der some conditions) and converting some of thesugar monomers to furfural and 5-hydroxymethyl-furfural.

The use of organic solvents presents two chal-lenges; the volatility and flammability of most sol-vents used requires rigorous procedures for theirsafe containment and for their recovery and recy-cle due to their cost.The carry-through of solventsto fermentation may also result in inhibition of bio-fuel production from biomass saccharides. Organo-solv pretreatment fractionates biomass into threeprocess streams; solid lignin, solid cellulosic fibers,and an aqueous hemicellulose stream. Figure 2provides a process flow for volatile solvents such asethanol or methanol. A catalyst, such as a mineral

acid, or Mg, Ca, or Ba chloride, sulfate, or nitrate,may be added to ethanol or methanol-based organo-solv delignification processes [31]. High-boilingpolyhydroxy alcohols (e.g., ethylene glycol andglycerol) permit the organosolv process to be con-ducted at atmospheric pressure, with high temper-atures causing delignification of the biomass. How-ever, there is a significant increase in energyrequirements for recovery of these high-boilingsolvents.

The Lignol process [32] was developed basedon an ethanol/water organosolv process, followed


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by conversion of the cellulosic fraction to sugars forfermentation to fuel. Typically the cooking stageis conducted at 180195C for 3090 min with3570 wt% ethanol and a liquid to biomass ratio of4:1 to 10:1 (wt/wt). The pH of the liquor is 2.03.8.The cellulosic fraction can be readily enzymatical-ly hydrolyzed in a short time (1224 h), and ligninis cleaved under the conditions employed in theprocess and dissolves in the hot aqueous ethanolsolvent. The addition of water to the black liquorstream precipitates the lignin, which is filtered,

washed, and dried.This lignin product has consid-erable value,and improves the overall process eco-nomics.The process continues to be developed byLignol Innovations, a Canadian company.

2.2.5 Ionic liquid pretreatmentIonic liquids of interest in biomass pretreatmentare organic molten salts with melting points typi-cally below 100C; many are liquids at room tem-perature.Their negligible vapor pressure and non-flammable nature have made them solvents ofchoice for green chemistry. In 2002, Swatloski et

al. [33] first reported that certain ILs were able tofully dissolve cellulose. In contrast, non-derivatiz-ing cellulose solvents such as transition metal com-plexes, cupram or cuoxam (aqueous cuprammoni-um solution), cuen (cupriethylenediamine hydrox-ide), and cadoxen (tetraethylenediamine cadmiumhydroxide) are not well suited for lignocellulosepretreatment because of their cost or toxicity. Sincethe first report, there has been considerable inter-est in the use of ILs to dissolve lignocellulosic bio-mass as a pretreatment method. While there are a

number of ILs able to dissolve cellulose at highconcentrations [34], fewer are able to dissolve lig-nocellulosic biomass at equivalent concentrations[35]. Both the anion and cation of the IL play a rolein lignocellulose dissolution. ILs containing imida-zolium-based cations contribute to lignocellulosesolubility via interactions with lignin phenolics[36]. The role of certain anions on lignin solubilityhas been studied with 1-butyl-3-methylimidazoli-um (Bmim)-based ILs [37];lignin solubility was ob-served to decrease in the order [MeSO4]

>Cl~ Br>>[PF6]

.The mechanism of cellulose and lig-

Figure 2. Process flowsheet of methanol or ethanol

biomass pretreatment (from Zhao et al. [31]).


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nocellulose dissolution has being explored experi-mentally [3840] and by MD simulations [4143].

Of the ILs able to dissolve lignocellulosic bio-mass, the most studied are those based on ethyl,methyl, or butyl imidazolium cations (Emim,Mmim, Bmim) with halides, acetate, or (methyl)phosphates as the anions. Halide anions providehigh solubility for lignocellulose provided the bio-mass is dry; the presence of water results in pre-cipitation of cellulose and a highly viscous mixtureis formed. Acetate, formate, dimethyl phosphate,and lactate anions can dissolve biomass containingtypical moisture contents (ca. 15 wt%). When ananti-solvent such as water, ethanol or acetone, ormixtures thereof, is added to dissolved biomass, thecellulosic fraction is precipitated and can be recov-ered by filtration or centrifugation. The resultingcellulosic fraction is very susceptible to enzymatichydrolysis [44, 45], exhibiting rates that are up toten-fold higher than are typical of other pretreat-ments [46, 47]. Figure 3 illustrates the advantagesof IL pretreatment [48].

An alternative to enzymatic hydrolysis of pre-cipitated cellulose is direct acid hydrolysis of cellu-lose in the IL. Addition of a mineral acid to cellu-lose dissolved in Bmim chloride at low tempera-

tures (around 100C) resulted in total reducing sug-ar yields of 5070% [49]. Recent results withaddition of HCl to corn stover dissolved in Emimchloride showed glucose yields approaching 90%[50]. Formation of byproducts still remains a chal-lenge, however.

Due to their current high cost, recovery and re-cycle of ILs used for pretreatment is required.Cel-lulose precipitation by anti-solvent requires regen-eration of near-pure IL to permit dissolution of fur-ther biomass. As they are high-boiling solvents,anti-solvent/IL mixtures require the anti-solvent

to be recovered by vaporization.Alternatively, withwater-miscible ILs, an aqueous biphasic systemmay be formed with kosmotropic anions, such asphosphates,present in the aqueous phase [51].Theresultant aqueous phosphate phase contains onlytrace amounts of IL.The aqueous/IL phase has re-duced water content, facilitating IL recovery.

2.3 Physicochemical methods

2.3.1 Steam pretreatmentSteam explosion pretreatment of biomass has itsorigin in the Masonite process, developed in late1920s for producing wood particle board [52].Woodwas chipped to less than 1 inch size, placed in agun and low pressure steam at 350 psig (190C)introduced for 3040 s. High pressure steam(1000 psig, 280C) was then introduced and thewood held for ~5 s prior to discharge through asmall discharge valve at the bottom of the gun.Thewood fibers passed into a cyclone, steam was sepa-rated and the fibers pressed to form Presdwoodor Quartrboard. As a common pretreatmentapproach today, biomass is exposed to steam at160260C for several seconds and discharged. Thefragmentation of the biomass by mechanical shear

during decompression substantially increases itssurface area.The variation of time and temperatureand biomass particle size in this process has beenextensively explored by Saddler and coworkers [8,53, 54]. Steam explosion produces degradationproducts,which together with acetic and other acids(formic, levulinic), are inhibitory to subsequent fer-mentation of the hydrolyzed saccharides.The steamexploded biomass must thus be washed prior to en-zymatic hydrolysis to remove these products.

Hemicellulose is rapidly solubilized in steamexplosion by acetic and other acids released.Lignin

Figure 3. Improved rates of biomass hy-

drolysis resulting from ionic liquid pretreat-

ment. Enzymatic hydrolysis of Miscanthus,

pretreated with [Emim][Ac] at 70C for 44 h

and precipitated with water (), 40 wt%

K3PO

4(O), or 40 wt% K

2HPO

4(). The hy-

drolysis of the pretreated substrates is com-

pared to the hydrolysis of untreated Mis-

canthus (). Data from [48].


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is melted, redistributed and repolymerized, whichsignificantly improves the rates of enzymatic hy-drolysis. A reduction in -5 and -O-4 aryl etherslinks has been reported with SO2 addition duringsteam explosion [8]. Addition of H2SO4 or SO2 canreduce the time and temperature required for

steam explosion, and decrease the formation of in-hibitory compounds. Steam explosion with andwithout the addition of catalyst has been tested andshown to be effective with hardwoods and agricul-tural residues; softwoods require acid addition. Ithas been operated at pilot scale by Iogen in Cana-da, and is arguably the pretreatment process clos-est to commercialization.

2.3.2 Ammonia fiber explosion (AFEX)The addition of liquid ammonia to biomass priorto steam explosion has been developed as an ef-fective pretreatment for agricultural residues [55].Typically, 0.51.0 kg of anhydrous ammonia per kgof dry biomass is added to pre-wetted biomass(60% moisture content) at 650 psi, and the systemheld at 130C for 15 min.The pressure is then re-leased through an exhaust valve, and the ammo-nia removed from biomass by air drying. A pro-posed process schematic for recovery and recycleof the ammonia is illustrated in Fig. 4. Only a smallamount of the biomass is solubilized in AFEX pre-treatment; little lignin or hemicellulose are re-moved.The AFEX process is most effective on bio-mass with low lignin contents (e.g., bermuda grass

5%, bagasse 15%) and less effective on woody bio-mass with higher lignin content (e.g., aspen chips25%). Dale and coworkers have extensively stud-ied the AFEX process, and optimum processingconditions for corn stover have been developed[56].

One recent development of the AFEX processhas been the ability to modify process conditions sothat pretreated biomass could be enzymatically hy-drolyzed and fermented without washing or in-hibitor removal. Yields of 191.5 g ethanol/kg un-treated corn stover, using enzymatic hydrolysis andfermentation by S. cerevisiae, were recently report-ed [57]. While degradation products from AFEXpretreatment decreased cell yields, they increasedspecific ethanol production, presumably due to in-creased maintenance requirements as reportedearlier [58].

Ammonia recycle percolation is similar to AFEXpretreatment. An aqueous solution of 1015% am-monia is recycled over biomass at 150170C forabout 15 min [59].The ammonia is then recoveredfrom the solution.The main effect of this pretreat-ment is cleavage of hemicelluloselignin linkagesand partial depolymerization of the lignin. Few in-hibitors are formed and a wash of the biomass isnot required. For both this process and AFEX, re-covery and recycle of the ammonia is critical to re-ducing the processing costs. In addition, there areconcerns related to the large-scale use of ammoniaat high pressures (see later).

Note:

25C, 6 atm

33C, 3 atm, V=0.4 66C, 3 atm, V=0.6

164C, 4 atm

20C, 21 atm

90C, 21 atm

164C, 4 atm

135C, 50% solids

T column < 140C at bottom

NH3column

3 atm

Ttop= 28C

Tbot= 135C

90C, 45% solids

10C, 3 atm, V=0

Biomass:NH3:water

NH4OH/NH3 NH4OH/NH3

Figure 4. Process flow diagram of an AFEX process using a quench ammonia recovery process (from Sendich et al. [101]).


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Table 1. Advantages and disadvantages of current pretreatment methods

Pretreatment method Advantages Disadvantages

Concentrated acid Almost complete hydrolysis of biomass Not all acid can be recovered

Low sugar loss Acid represents a major cost

Low temperature

Dilute acid Hydrolysis of hemicellulose Needs high temperature (~160180C)

Low residence time (510 min) Sugar degradation produces inhibitors of fermentation

Base Low temperature High residence time (days)

Low sugar loss Bases cannot be recovered and re-used

Feedstock flexible Use of bases is costly

Organosolv Lignin can be recovered Solvents are volatile and flammable, and inhibit fermentation

Feedstock flexible High temperature may be needed (up to 250C)

Use and recovery of solvents is costly

Ionic liquids Lignin can be recovered Solvents can inhibit fermentation

Feedstock flexible Use of these solvents is costly

Solvents are not volatile

Susceptibility of resulting cellulosefraction to enzymes is relatively high

Steam explosion Short residence times (


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the degree of cellulose crystallinity, although ARPis typically more effective in this regard when ap-plied to agricultural residues such as corn stover[68]. It has been demonstrated that AFEX pretreat-ment significantly increases polysaccharide acces-sibility to saccharolytic enzymes by disrupting theester linkages and other bonds that comprise thelignin carbohydrate complex [70]. Typical glucanyields of AFEX pretreated biomass, such as agri-cultural residues, are between 80 and 85% and xy-lan yields can be as high as 9598% when the cellu-lase cocktails are augmented with hemicellulases[70].

As it does not remove the biomass during pre-treatment, AFEX does not significantly alter the

physical structure of the biomass compared to oth-er techniques where partial and/or complete solu-bilization occurs. In a recent comparative study,there was evidence that lignin may be redepositedon the surface of the biomass after AFEX pretreat-ment in a similar fashion to dilute acid [68]. Thisfinding is in agreement with a prior report thatfound carbon rich components at the surface of thepretreated biomass after AFEX [71]. Results fromthe same comparative study indicate that biomasspretreated with the ARP technique possess holeson the surface of the biomass and that there is dis-

ruption of the biomass network. These results areconsistent with previous results that show thatARP effectively removes lignin and hemicellulose[72].

3.2 Organosolv pretreatment

The organosolv pretreatment technique is an ef-fective technique to disrupt lignin and generate aproduct with significantly lower lignin content.Theorganic solvent enhances biomass delignificationby promoting the fragmentation and dissolution oflignin. The selection of the organic solvent and aparticular catalyst offers the possibility of optimiz-ing the pretreatment process over a range ofprocess conditions and targeted product character-istics (e.g., readily hydrolyzed vs. complete deligni-fication) for specific feedstocks. In general, it is ac-cepted that primary alcohols are better pretreat-ment agents than the secondary or tertiary alcoholsin terms delignification efficiency, and have the ad-vantage of being easier to recover and available atlower costs.

There are several examples highlighting the ef-ficacy of primary alcohols as a pretreatment sol-vent. For example, Loblolly pine has been shown tobe effectively disrupted by ethanol and H2SO4 asthe catalyst via acid-catalyzed cleavage of -O-4linkages and ester bonds within the lignin matrix[73]. The residual and dissolved lignin present af-ter this type of organosolv pretreatment wereshown to have significantly higher levels of car-boxylic acids, phenols,and decreased aliphatic car-bon content [73]. Another recent study comparedthree different catalysts H2SO4, NaOH, and MgCl2 with ethanol as the organic solvent for the pre-treatment of Pinus rigida. It was found that a 1%H2SO4 loading generated a product with yields be-tween 55 and 60%, whereas a 1% MgCl2 loadingachieved nearly 60% yields. NaOH at a 1% loadinghad little or no effect, but increasing the loading to2% resulted in yields of 80% [74]. From a structuralperspective, it has been reported that the ethanolorganosolv pretreatment of B. davidii selectively

removed lignin from the middle lamella of the plantcell wall without disrupting the crystal structure ofthe cellulose, and enhances enzymatic hydrolysis.This pretreatment process significantly deformsthe plant cell wall and produces breaks and cracks[75]. Similarly, methanol in combination withH2SO4 and H3PO4 as catalysts has been shown to bean effective organic solvent for the pretreatment ofpoplars under certain process conditions with glu-cose yields between 70 and 88% [76].The pretreat-ment of pine with 6080% aqueous methanol(MeOH) solution with HCl as the catalyst removed

Figure 5. SEM image (10 000) of purified lignin preparation after pre-

treatment with 0.8% H2SO

4at 170C; pretreated lignin was deposited

onto Whatman #1 filter paper for SEM imaging. Droplet formations

produced from a Jack Pine lignin preparation. Image adapted from Selig

et al. [69].


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~75% of the original lignin, and pretreated beechreached a level of 90% delignification [77].

3.3 Ionic liquid pretreatment

Depending on the anioncation pair and the pro-

cess conditions employed, ILs can have substantialimpacts on the chemical and physical structure ofthe biomass over a wide range of feedstocks (i.e.,wood, agricultural residues, and grasses). The fun-damental parameters that determine IL pretreat-ment efficiency have been investigated, and a re-cent study evaluated the relationship between theKamlet-Taft , , and * solvent polarity parame-ters of three different ILs [Emim][OAc], [Bmim][OAc], and [Bmim][MeSO4] and correlated theseparameters to performance. It was found that forthese selected compounds the parameter is anexcellent predictor of IL pretreatment perform-ance as the ILs with [OAc] ( > 1.0) as the anion re-move >32% of lignin from maple wood flour and re-duce cellulose crystallinity, whereas [MeSO4]

( = 0.60) removes 19% of the lignin with no de-crease in crystallinity [78].

Chemical changes induced by the IL pretreat-ment using [Emim][OAc] on samples ofE. globulusinclude deacetylation of xylan, acetylation of the -aryl ether linkages in lignin, and the selective re-moval of guaiacyl units that increase the S:G ratio.This pretreatment significantly reduced theamount of crystalline cellulose present and in-duced a transition from the cellulose I polymorphto cellulose II. The pretreated material exhibitedsignificantly higher enzymatic saccharificationyields. The plant cell walls of E. globuluswere ob-served to undergo significant swelling after expo-sure to [Emim][OAc] [79]. [Emim][OAc] was alsoobserved to rapidly swell the plant cell walls ofpoplar at room temperature[80].Using the same ILto pretreat switchgrass resulted in similar chemicalchanges, but the pretreatment was able to com-pletely solubilize the plant cell walls at elevatedtemperatures [81].

4 Economic assessment of pretreatmentmethods

4.1 Importance of economic assessment

Pretreatment represents the major cost in lignocel-lulosic biofuel production through the biochemicalroutes envisioned today [8284]. Economic assess-ment of biofuel processes can help quantify the im-pact of different technologies under development.As there are many pretreatment technologies and

because they have a common economic goal pro-viding inexpensive sugar cost analysis of the dif-ferent options seems particularly fitting.

Although experiments are the source of dataused to construct models that project the perform-ance of different processes, experimental data

alone is insufficient to make such projections.Themain reason for this is that experiments on onepart of the process, for example, pretreatment, arealmost always carried out in isolation of other partsof the process, for example, fermentation and prod-uct recovery. However, these processes will be in-terconnected in a commercial setting, and thus oneshould account for the interactions between sub-processes during the assessment exercise.Further-more, the scale of an experiment does not neces-sarily reveal factors that emerge at larger scales.Technoeconomic analysis, i.e., the quantification ofthe technical and economic metrics that define aprocess, aids in solving this scale-dependent sys-tems-level problem.

Technoeconomic analysis of chemical process-es is a mature field with various applications, en-compassing activities in process modeling, processdesign, feasibility analysis, decision-making underrisk, and research evaluation. It has been applied,with various degrees of depth and thoroughness, topretreatment technologies [63, 84, 85]. Here, in-stead of ranking the different technologies on eco-nomic grounds, we try to provide insights on dif-ferent ways in which pretreatment can influencethe cost of the resulting product. We do not list theproperties of an ideal technology, since an ideal isby definition unattainable in practice, but merelystate the cost drivers and the tradeoffs that arisedue to the interplay of the characteristics of differ-ent pretreatments.

4.2 Cost drivers

The choice of pretreatment method can influencethe production cost in several ways,both directly atthe pretreatment stage and indirectly at other partsof the process. Generally, major cost drivers influ-

enced by pretreatment are related to equipment,energy, material, and waste treatment costs. Otherfactors influenced by pretreatment, albeit more in-direct in nature, could affect the process econom-ics, including the availability of useful by-productsand the need for special safety measures. Thesecost drivers will be briefly discussed.

4.3 Equipment

Equipment purchase and installation is the most ob-vious component of the capital cost of a production


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facility. The cost of equipment is derived both fromthe material needed for its construction and fromthe engineering capabilities needed to design it andassemble it. Different materials of construction areused depending on the chemicals to be handled by apiece of equipment and on the conditions of opera-

tion, and their purchasing prices can vary consider-ably (Table 3).The size of the equipment, related tothe throughput and residence time, determines theamount of material needed for construction andthus affects the capital cost. The cost, in general,varies less than linearly with volume (


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Biotechnol. J. 2011, 6, 10861102


loading, low inhibitor concentration, etc.) result inoverall lower energy use.

Pretreatment processes can have significantlydifferent energy requirements. A helpful illustra-tion is provided by Eggeman and Elander [85], whocompared the energy uses of processes based on

various pretreatment methods. For example, diluteacid uses less than half the steam needed for hotwater pretreatment, not only because less energy isrequired at pretreatment when using acid, but alsobecause the resulting hydrolyzate is more concen-trated. A more concentrated hydrolyzate results ina higher concentration of ethanol after fermenta-tion, and thus a lower steam use during the distil-lation used for product recovery. Electricity use canalso differ with the choice of pretreatment method.The same study found that AFEX uses ~50% moreelectricity than ARP. These methods are based onthe same physicochemical principles, but the dif-ferent operating conditions dictate changes in theenergy balances for the respective processes.

4.5 Materials and waste disposal

Most pretreatment methods use chemicals or cata-lysts that can contribute significantly to the rawmaterial cost.These chemicals include acids,bases,peroxides, salts, and pressurized gases, and thepossibility for recycling these is often listed as amajor advantage precisely because their use canaffect the cost appreciably. Several chemicals areknown to perform well at delignifying biomass andincreasing enzyme digestibility, but they are usual-ly not extensively considered because of economicconsiderations.Such is the case of H2O2/NaOH andperacetic acid treatments, which employ expensivechemicals and are hard to recycle [93].Cost limita-tions are also at play when considering ammonia-and IL-based methods [35, 63].

The choice of pretreatment can also influencethe use of raw materials elsewhere in the process,most notably in saccharification. Because the costof enzymes makes up a major fraction of the rawmaterial costs, this influence can have a marked ef-

fect on cost. Therefore, pretreatments that use ex-pensive chemicals but that deliver an easily-di-gestable material can still be economically attrac-tive. An example of this interplay is IL pretreat-ment, which requires an expensive chemical and arecycling process but results in a feedstock that iseasily saccharified [94].

When the catalysts or chemicals used in pre-treatment cannot be fully recycled, their disposal isrequired. Such disposal represents itself a cost,consisting on inactivation of the chemical so that itcannot harm the environmentwhich usually re-

quires additional equipmentand subsequent re-moval of the facility by transportation or sewage.One example is the case for dilute acid pretreat-ment. The process uses sulfuric acid during pre-treatment and must be discarded subsequent to it.This in turn requires neutralization, often with

lime, which produces calcium sulfate (gypsum). Atypical process can generate nearly 33 kg of gyp-sum per tonne of feedstock processed, which at ascale of 2000 MT/day translates into 2.5 MT/h ofgypsum waste [87], or about two to three full semi-trailers per day.

4.6 Indirect costs

Lastly, the pretreatment choice can have indirecteffects on the cash flow. Since pretreatment islargely responsible for separating the biomassfeedstock into its constituent components, it is alsoresponsible for dictating what by-products can re-sult from the process and their degree of purity.Specifically, and because lignin is non-ferment-able, this biomass component has the largest po-tential for becoming a revenue source, either forthe production of electricity or, potentially, as afeedstock for the production of specialty chemicals[95]. If the lignin is separated and purified, it maybecome an essential part of the process economics.At a selling price in the range of $35/kg, the ligninrevenue is high enough to make this compound themain product of the plant, with the fuel becomingthe by-product. A similar trend has been shown tooccur in algal biofuel processes, where nutraceuti-cals and other fine chemicals could be sold to sub-sidize the cost of making the fuel [96]. However, atthe large scales envisioned for biofuels, the mar-kets for by-products can easily saturate, causing asharp decrease in price and counteracting their ef-fect in biofuel economics.

Other indirect effects that result from the choiceof pretreatment are the overhead expenses. Oneexample is process safety, which dictates measuresthat range from specialized equipment and labor tolegal fees for permitting and accident insurance.

Federal and state laws require process equipmentto be insured against failure and the damage thatcan ensue, and the regulations are extensive (see,e.g., [97]). Insurance is costly and deductibles canbe high,and accidents themselves can cost take notonly large sums of money (in the order of hundredsof millions of dollars [98]), but also human lives.One example of a toxic chemical used for pretreat-ment is ammonia, which can be lethal in a matterof minutes when concentrations are high [99]. Inone example, in Colombia, USA (1977), a suddenrelease of ammonia killed 30 and injured more


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than 20 [98].Although hard to quantify, safety con-cerns can add to the costs of an operation throughthe perceived risk of a process, which may limit theaccess to capital funding. Regardless of these facts,one should not discard a pretreatment method be-cause of safety concerns alone, as hazards can be

contained and the benefits can in many cases out-weigh the costs. Ammonia production is actually agreat example of this: the majority of the agricul-tural industry depends on this process.

4.7 Conclusions on the cost of pretreatment

As stated before, the goal of pretreatment/sacchar-ification is to produce cheap sugars that can be ef-ficiently processed to a fuel or chemical. This im-plies that maximization of yield is the most impor-tant parameter when analyzing the deconstructionof biomass in isolation, but others become obviouswhen analyzing it in the context of the rest of theprocess. Although the goal of pretreatment is in-creasing the sugar return on capital and operatinginvestment, it is also important to do so in a man-ner that is cost effective when considered it as partof a biorefinery.

Because of that, researchers should be aware, ingeneral terms, of the factors that can influence thecost of a process.Although it is impossible to list allsuch factors in a few paragraphs, some of the mostrelevant factors are given in the preceding sections.From the above discussion it is obvious that most ofthe issues that ultimately affect production costsare not considered at the experimental level. Ex-periments are carried out in small scales, usually inglass vessels,employing pure and relatively expen-sive chemicals and without considering the heat orpower consumption, and in controlled environ-ments where the risk of accidents can be efficient-ly minimized. Understanding the cost drivers,therefore, allows the experimenter to address is-sues that are likely to become impracticable atcommercial scales in a timely fashion.

This work conducted by the Joint BioEnergy Institutewas supported by the Office of Science, Office of

Biological and Environmental Research, of the U.S.

Department of Energy under Contract No. DE-AC02-

05CH11231.

The authors have declared no conflict of interest.

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