Cellulose and Biomass ConversionTechnology and Its Application toEthanol Productionfrom CornDR. C.E. WYMANNational RenewableEnergy Laboratory

].ansportation fuels are almost

totally derived from petroleumand accounted for more than

one-quarter of the total energy used inthe U.S. in 1989, more than 81 quads.'Currently, almost 50% of all petroleumused in the U.S. is imported, and im-ports have risen markedly over thelast few years.' Because OPEC con-trols about 75% of tbe world's oil re-serves, and the U.S. has only about5% of that total, it is likely that petro-leum imports will continue to rise.

Few substitutes exist for petro-leum-based transportation fuels, andthe U.S. is extremely vulnerable strate-gically and economically to disrup-tions in its supply, as illustrated by ourexperience with Iraq's invasion ofKuwait. In addition, imported petro-leum contributed about 40% of thebalance-of-payments deficit for theU.S. in 1989'

Evaporative losses during fuel-ing of automobiles, losses from thefuel system, and nitrogen oxides andunburned hydrocarbons from auto-motive exhaust contrihute [0 ozoneformation in many cities, such as LosAngeles.' High altitude cities, such ~IS

Denver, experience excessive levelsof c.ubon monoxide because uf in-complete comhllstion of carbon-con-iuining luels, Petroleum derived trans-porration fuels are rhe source of lip totwo-i hirds ()I carhun monoxide andone-third to one-half of smog c:lu:iing

MARCH/APRil J 993

SSF fermentation.

fUEl REfORMULATION 67

em i:<sion~.·\"t.'·"For these reasons .md others. in-

terest in ethanol - und ethanol madefrom biorna s-, - <15 a transportationfuel has increased.

Ethanol is :1 clean-burning liquidfuel that em be readily substituted forg,lsoline in our rransportanon sector.When ethanol is produced from re-newable sources of cellulosic biomass.ethanol can improve energy security,reduce [he balance-of-payments de-ficit, decrease urban air pollution. en-hance agricultural income, and reducethe accumulation of carbon dioxide inthe atmosphere.Y" 10

The Ethanol from Biomass Pro-gram. managed by the National Re-newable Energy Laboratory (NREL) forthe Biofuels Systems Division of theU.S. Department of Energy (DOE), isdirected at lowering the cost of ethanolproduction to the point at whichethanol can compete with gasolinewithout tax incentives, so that the ben-efits of this unique fuel can be morewidely realized. In this overview,technology for ethanol productionfrom cellulosic biomass will be de-scribed. Opportunities for applyingthis technology to increase revenuesfrom dry and wet milling processes forethanol production from corn will alsobe discussed.

Cellulosic BiomassCellulosic biomass is a complex

mixture of carbohydrate polymersfrom plant cell walls known as cellu-lose and hemicellulose, plus lignin andsmall amounts of other compoundsknown as extractives (see Figure 1).Examples include agricultural andforestry residues, municipal solidwaste (MSW), herbaceous and woodyplants, and underutilized standingforests. Cellulosic biomass is inexpen-sive to produce.

The cellulose fraction is com-posed of glucose sugar moleculesbonded together in long chains thatare held together in a crystalline struc-ture. The hemicellulose portion of bio-J1l:.lSS is made of long chains of a nurn-her of different 5118;]rs and does notliuve " LTY~l~lllint;!structure. For hard-woods and m:lny gr3sscs, the predom-inant component of hemicellulose isxylose. a five-carbon sugar that hashistorically been more difficult to con-

68 FUEL REFORMULATION

vert into useful products than glucose11:1" been.

For the U.S. it is estimared tharthe total amount of collectable under-utilized wood, :Igricultural residues.short-rotation energy crops. and i\ISWcould provide about 2,700 million drytons of cellulosic biomass per year atprices from $20-70 per dry ton." 10 Thisquantity of feedstock can generateabout 300-billion gallons of ethanol,more than double current U.S. gasolinemarket demand of 140-billion gallonsof ethanol equivalent.

The price of these r3W materi-als is low enough to provide a reason-able margin for conversion intoethanol. Even though the magnitudeof the resource is subject to significantuncertainty, the potential availability ofrenewable feedstocks is projected tobe substantial while the cost is reason-able. Thus, cellulosic biomass is a fa-vorable feedstock for fuel ethanol pro-duction.

Ultimately, abundant feedstocksare required to achieve large-scaleethanol production. Oak Ridge Na-tional Laboratory (ORNL) manages aBiomass Production Program for DOEto develop technology for producingfast-growing herbaceous and woodycrops that will provide an abundantand low-cost substrate for ethanol pro-duction,

Conversion of CellulosicBiomass into EthanolCellulosic biomass is an inex-

pensive feedstock. and acids or en-zymes wil! catalyze the breakdown oftile cellulose and hemicellulose chainsinto their component SUg:lf molecules.which can be fermented into ethanol.Lignin is a complex phenolic polymerthat cannot be fermented into ethanol.The challenge is to develop low-costmethods to convert the naturally resis-rant cellulose and hemicellulose intoethanol.

Over the years. J number ofprocesses have been studied for con-version of cellulose-containing bio-mass into ethanol through reactionscatalyzed by dilute acid, concentratedacid, or enzymes known as cellulases.In each of these options, the feedstockis pretreated to reduce its size and fa-cilitate downstream processing, asshown in Figure 2. The cellulose frac-tion is hydrolyzed by acids or enzymesto produce glucose sugar, which issubsequently fermented to ethanol.The soluble xylose and other sugarsderived from hemicellulose can alsobe fermented to ethanol, and the ligninfraction can be burned as fuel topower the rest of the process, con-vetted into octane boosters, or used asa feedstock for the production ofchemicals.

Acid-Catalyzed ProcessesSeveral dilute acid hydrolysis pi-

lot plants were constructed in the U.S.during World War II as part of an effortto produce ethanol for fuel use," butthe economics were too unfavorable to

Figure I. Cellulosic biomass consists of cellulose, hernicellulose, lignin, and someextractives as shown here for representative examples of agricultural residues.

Hemicellulose 32%

Cellulose 38%"'-"-"-r.Lignin 17%Other 13%

Agricultural Residues

Other 22%Metal 10%Food ond yord woste 13%Wood 2%Plastics and textiles 7%Paper 46%

Municipal Solid Waste

Cellulose 50%Hemicellulose

23%

Lignin 22%

Underutilized and Short Rotation Hardwoods

Cellulose 45%

Hemicellulose30%

Lignin 15%Other 10%

Herbaceous Energy Crops

MARCH/APRil 1993

allow continued operation in ~t freemarket economy. Dilute acid-catalyzedprocesses are currently operated inEasre rn Europe for converting cellu-losic biomass into ethanol and singlecell protein Low yields of 'i0-70% typ-ical of dilute acid sy.stems make theseprocesses unable to compete with ex-isting fuel options." 13

Concentrated sulfuric or halo-gen acid options achieve the highyields required. However, becauselow-cost acids, (e.g., sulfuric) must beused in large amounts and more po-tent halogen acids are relatively ex-pensive, recycling of acid by efficient,low-cost recovery operations is essen-tial to achieve economic operation. It Ij

The acids must be recovered at a costsubstantially lower than that of pro-ducing these inexpensive materials inthe first place - a difficult require-ment. Several processes are being de-veloped for concentrated-acid hydroly-sis of biomass.

Enzyme-CatalyzedProcessesEnzyme-catalyzed processes

achieve high yields under mild condi-tions with relatively low amounts ofcatalyst. Enzymes are also biodegrad-able and environmentally benign.Over the years, several enzyme-basedprocesses have been studied at thelaboratory scale, but only a few inves-tigations have been expanded to alarger scale. The three leading pro-cesses considered are discussed be-low.

Separate Hydrolysisand FermentationIn the separate hydrolysis and

fermentation (SHF) process, the cellu-losic biomass is first processed in apretreatment device to open the bio-mass structure. A portion of the pre-treated biomass is used in an enzymeproduction vessel to support growth ofa fungus that produces cellulase en-zyme, and the cellulase is added to thebulk of the pretreated substrate in ahydrolysis reactor. At this stage, theenzyme catalyzes the breakdown ofthe cellulose hy a so-called hydrolysisreucuon to term glucose sugar. Tilestream from the Ilyclrulysis processp~lsses on to a fermenter ro whichyeast are added to convert the glucose

MARCH/APRil 1993

Figure 2. The process flow diagram for acid- or enzyme-catalyzed conversion ofcellulosic biomass to ethanol.

Xylosefrom Hemicellulose

into ethanol. Finally, the ethanol isseparated from the rest of the fermen-tation broth in a purification step. 16,17

SimultaneousSaccharification andFermentationThe sequence of steps for the si-

multaneous saccharification and fer-mentation (SSF) process is virtually thesame as for separate hydrolysis andfermentation except that hydrolysisand fermentation reactions are com-bined in one vessel. 18. 19 The presenceof yeast along with the enzymes mini-mizes the accumulation of sugar in thevessel, and because the sugar pro-duced during breakdown of the cellu-lose slows down the action of the cel-lulase enzymes, higher rates and yieldsare possible for SSF than for SHF. Ad-ditional benefits come from elimina-tion of about half of the expensive fer-mentation vessels and a mixture thatis less vulnerable to invasion by un-wanted microorganisms due to itsethanol content.

DOE/NREL has focused on theSSF process as a promising route toachieve low-cost fuel ethanol produc-tion within a reasonable time frame."

Direct MicrobialConversionThe direct microbial conversion

process combines [he enzyme produc-tion, cellulose hydrolysis, and sugarfermentation steps in one vessel. 21. 2123

In the most extensively tested con fig-urauon, two bacteria are employed toproduce cellulase t'JlzYlTles ariel fer-merit the sugars formed hy breakdownof cellulose and hemicellulose into

Ethanol

BeerLignin

ethanol. Unfortunately, the bacteriaalso produce a number of products inaddition to ethanol, and yields arelower than for the SHF or SSFprocesses. Due to the simplicity of theprocess, the DMC option is verypromising if yields and concentrationsof ethanol can be improved.

Pretreatment ofCellulosic BiomassCellulosic biomass is naturally

resistant to enzymatic attack, and apretreatment step is required to over-come this resistance if the enzyme-cat-alyzed hydrolysis is to proceed at areasonable rate with the high yields vi-tal to economic viability. Several op-tions have been considered for bio-mass pretreatment, including steamexplosion, acid-catalyzed steam explo-sion, ammonia fiber explosion,organosolv, and dilute acid.

The dilute acid option has goodnear-term economic potential." In thisprocess, about 0.5% sulfuric acid isadded to the feedstock, and the mix-ture is heated to about 140-160Q C for5-20 minutes. Under these conditions,most of [he hemicellulose is brokendown to form xylose sugars, leaving aporous material of primarily ceJJuloseand lignin that is more accessible forenzyme attack. Evaluation of the e1iluteacid process with various agriculturalresidues, short-rotation hardwoods,and herbaceous energy crops has con-sistently shown that the conversionyields correlate well with the degree ofhemicellulose removal." Although {hisprocess has good near-term potential,

signific.mt benefit would result if alow-cost scheme could be devised that

FUEl R£FORMUlATION 69

nOlild ;ti:-iOre-move lignin hcclu,ie the

:mlid lignin ;1::i;ioci,l[cdwirh the ccllu-IIl.'iL· lTL';lle:; .soruc pron::;:-iing difficul-tie:; in rhl:' hydroIY:ii:i ;llld fermentation,;tcps,

Hemi,ellulose UtilizationTile hemicellulose polymers in

cellulosic biomass such ;IS hardwood..rgriculrura l residues. .md herbaceousplants c.m he readily broken down [0

form five-carbon sugars such as xyloseduring the pretreatment step, How-ever. until recently, the xylose streamcould not he utilized, and it was nec-essary [(J send this materia! [(J wastedisposal. From an economic perspec-rive, this costs the process twice: first,in payment for the xylose, and secondin disposal costs,

Several options, outlined below,have been examined for xylose utiliza-tion:

Conversion of Xyloseinto FurfuralFor dilute acid-catalyzed break-

down of cellulose to fermentation sug-ars, a significant fraction of the xylosewill degrade into furfural.'!> Similarly,the xylose released by pretreatmentcan be reacted [0 furfural. This prod-uct is currently manufactured for usein foundry and other applications, so itcould be sold as a by-product, gener-ating additional revenues. However,the furfural market would be quicklysaturated by the volume of furfural thatwould accompany large-scale produc-tion of ethanol as a fuel/ limiting thenumber of ethanol plants that furfuralsales could su pport,

Yeast for EthanolProductionCertain strains of yeast are

known to ferment xylose into ethanol,such as Candida shehat.ie, Pichia stipi-tis, and Puchysolen tannophilus,LH,2<),'<IJI

However. these strains require smallamounts of oxygen in the fermentationbroth to ferment xylose (micro-.ierophil really). La rge-scale productionof ethanol fuels will probably requirethe use of hUHe fermcnters with V(ll-

urnes :lppro:Il'l1ing :1 million gallonseach, and proper control of oxygen insuch brHe vesse ls c()ldd he challeng-in,Q, FuI'tllerrnol'e. these yeast strainstypically cannot yet achieve very high

70 fUEL REFORMULATION

ethanol production rates or toleratehigh ethanol concentrations .'!

Other Microorganisms forEthanol ProductionOther microorganisms, such ;IS

thermophilic bacteria and fungi, cananaerobically ferment xylose intoethanol. "-"i.,\i.,ln,l-.l.' However. ethanol tol-

erunce has not been satisfactorilydemonstrated for bacteria, althoughsome new evidence suggests that pre-vious conclusions may have been pre-mature, ''l New information indicatesthat the yields could be improved incontinuous culture." The fungi evalu-ated suffer from similar limitations inboth ethanol tolerance and yield,

SimultaneousIsomerization andFermentation of Xyloseto EthanolSeveral groups have studied the

use of xylose isomerase enzyme toconvert xylose into an isomer calledxylulose that many yeasts can fermentinto ethanol under anaerobic condi-tions ."l.41 Researchers at NREL geneti-cally engineered the common bacteriaEscherichia coli to produce large quan-tities of xylose isomerase for such aprocess, and ethanol yields of 70% oftheoretical have been achieved in thesimultaneous isomerization and fer-mentation of xylose process." In thisconfiguration, the enzyme and yeastare employed together to drive theequilibrium-limited fermentation tocompletion, with the primary yield lossresulting from xylitol formatton.t':"This process has the advantage of em-ploying anaerobic yeast that are easierto use on a large scale, but the needto provide xylose isomerase enzymeand adjust for differences in pH optimabetween the yeast and enzyme com-plicate the technology,

Genetically AlteredBacteriaResearchers at the University of

Florida have successfully introducedthe genes from Zymo mon as mobilisinto the common bacterin m F.. colt,«lebstet!« uxytcica. and or hers ~o thatthey can now ferment xylose directlyinto ethanol. '(",-,""),;"This approach has{he advamage that a single orga nisrnGill carry out the fermentation of xy-

lost'. and initial data suggest that highyields .ire possible, These buctertu cur-rentlv require operation at ne.rr-neutr.ilpH wh ilc production of bvproducr;Icic.ls tends [0 drive the pH down. thusrequiring addition of bases Invaxion

hy other bacteria could be problem-.itic.

At this time, promising optionsfor xylose conversion are the use ofgenetically engineered bacteria andmicroaerophilic yeast, However. fullintegration of these technologies intothe overall conversion process is re-quired to evaluate and improve theirperformance, Advantages would alsoresult if these options could be im-proved further by genetic modifica-tions or other approaches,

Lignin ConversionAs shown in Figure 1, lignin

generally represents the third largestfraction of cellulosic biomass and isnot significantly different in quantitythan hemicellulose. Thus, it is impor-tant to derive value from the ligninfraction if the economic production of'ethanol from biomass is to beachieved. Three options, discussed be-low, lead the possibilities for ligninuse:

Lignin as a Boiler FuelLignin has a high energy COIl-

tent and can be used as a boilerfuel. ;),2,;3 The amount of lignin in mostfeedstocks is more than enough tosupply all the heat required for the en-tire conversion process and to gener-ate enough electricity to meet its de-mands, In fact, excess electricitybeyond all of these needs is generated,and additional revenue can be gener-ated from electricity exports from theplant. The electricity sold for currentplant designs is equivalent (0 about 8%

of the energy value of [he ethanolproduct, and greater revenues arelikely as the technology is improvedto require less process heat and elec-tricity,

Producing OctaneBoosters from LigninLignin j, a complex phenolic

polymer that can be broken down toform :l 111ixture of monomeric phenol iccompounds and hydrocarbons, Thephenolic fraction can he reacted with

MARCH/APRIL 1993

alcohols to form methyl or ethyl arylethers. which are good octane boost-ers.I., Be cau se gasoline additives are

more valuable than boiler fuel. [his op-rion for lignin Lise would generatemore revenue. However, the technol-ogy must be improved to provide highproduct yields, .inrl the conversioncosts must be low enough to provide anet income gain for the ethanol plant.

Producing Chemicalsfrom LigninA number of chemicals could be

produced from lignin including phe-nolic compounds, aromatics, dibasicacids, and oleflns." Such materialscould have a high value that wouldaugment the total revenue for theethanol plant. However. just as for theconversion of lignin into octane boost-ers, the cost of the conversion processmust be low enough to ensure a netgain in revenue. In addition, highyields of target products will likely benecessary to achieve economic viabil-ity, and significant markets must existto be compatible with large-scaleethanol production.

Progress and PotentIalfor ImprovementProgress on enzyme-catalyzed

processes to convert cellulosic biomassinto fuel ethanol has been substantialduring the last 10 years, with projectedselling prices dropping from about$3.60/gal. in 198056 to only about$1.27/gal. now." This reduction in sell-ing price is a result of improved ratesand yields from the SSP process, im-provements in enzymes to achievehigh yields with lower loadings, selec-tion of better fermentative microbes,and advances in xylose fermentationsthrough genetic engineering.

Although research progress hasbeen substantial, significant opportuni-ties still exist to reduce the sellingprice of etha nol from cellulosic bio-mass to S.67/gal. at the plant gate.Key target areas include further im-provements in glucose and xyloseyields from pretreatment; increasedethanol yields to 90% or greater fromcelllllo,~e ane.! xylose fermentations, de-creased stirring and pretreatmentpower requirements: better producuvt-tics through continuous processingand l)iul~llalys[ iunuohilizat iun; low-

72 fUEl REfORMUlATION

cost production of octane enhancers orchemicals from lignin: increasedethanol concentration: and reductionof fermentation limes. Because feed-stock costs are a significant fraction ofthe final product selling price, im-provements in feedstock production,collection, and genetics could provideadditional cost reductions througheconomies of scale for larger ethanolplants, decreased feedstock costs. andless nonfermentable feedstock. Manyof these goals have been met individ-ually; the evidence that the rest can beachieved is great. The primary needis to meet these goals simultaneously.It is also encouraging that enough op-tions exist to lower the selling price ofethanol that not all the technical goalsmust be achieved to reach the targetselling price.

Applications to CornEthanol ProductionThe cost of the feedstock is an

important portion of the projected sell-ing price of ethanol from cellulosicbiomass. At the current projected$1.27/gal. selling price, $0.46 is forfeedstock at a price of $42/ton deliv-ered to the plant gate. For example,if a free feedstock could be obtained,ethanol could be produced for about$0.81/gal. Because ethanol from cornnow sells for about $1.20-1.30/gal, useof an inexpensive feedstock could beadvantageous. Several possible low-value or waste streams could prove at-tractive, including the carbohydratefractions of corn gluten feed from cornwet milling; distillers' dried grains andsolubles (DDGS) from dry milling fromcorn; agricultural waste streams suchas corn cobs, corn stover, or wheatstraw; commercial processing wastestreams such as white water in papermanufacture; and domestic wastessuch as MSW. Although such feed-stocks are of limited availability, theyprovide an opportunity to establish thetechnology early.

Utilization of the FiberFrom Wet MillingProcessesAbour two-thirds of the ethanol

made from starch in rile U.S. is madeI)y wet milling processes.'·'" Tvpicallvabout 2.5 gal. ofetl1anol, .I31b of ananimal feed coproduct called corn

gluten feed, 3.4 Ib of .inorher a nirna lfeed coproduct called corn glutenmeal. and 1.7 Ib of corn oil are pro-duced from a bushel of corn (-l7.3 lbdry weight). Corn gluten feed. un ani-mal feed containing 21% protein, re-sults from combining three by-productstreams: fiber, germ meal, and cornsteep liquor. The fiber and germ mealproduct streams contain significantamounts of cellulose, hemicellulose,and starch.

The price of corn gluten feedand other animal feeds seems to be setlargely by their protein content, givingless value to the cellulose, hemicellu-lose, and other carbohydrate materials.In fact, since most of the corn glutenfeed is sold to the European EconomicCommunity, the cellulose, hemicellu-lose, and other carbohydrate materialsadd to shipping costs. A process thatcould convert cellulose, hemicelluloseand other carbohydrates into valuableproducts such as ethanol could en-hance the revenue for the plant whilereducing most shipping costs.

The composition of the fiberstream is about 22.5% starch, 12.5%protein, 3% fat, 10% cellulose, 30%hemicellulose, 0.5% lignin, and 2%ash. The germ meal contains 26% pro-tein, 20% starch, 5% fat, 13% cellulose,32% hemicellulose, 1% lignin, and 4%ash. From every bushel of corn, 5.41bof fiber and l.9 Ib of germ meal areproduced. Thus, from one bushel ofcorn, these combined streams contain2.2 Ib of hemicellulose, 0.79 lb of cel-lulose, and l.0 lb of starch."

These streams could be con-verted into 0.26 gal of ethanol fromhemicellulose and cellulose, and 0.13gal of ethanol from starch, or a total of0.39 gal of ethanol, an increase of 16%over the quantity of ethanol typicallyproduced from a bushel of corn. Thisincreased production would increaseethanol revenue by 16% while de-creasing shipping costs of corn glutenfeed by as much as 42%. For every 1billion bushels of corn pro-cessed, to-tal ethanol production from the carbo-hydrate fraction of the fiber and germmeal streams could be 400 million gal-lons. which represents revenues of5480 million at an ethanol price of:5120/~pl.

On the other hand, tile proteincontent of the corn gluten feed would

MARCH/APRil J 993

·,I

figure 3SSFprocess: Endo- and exo-glucanaseplus beta-glucosidase enzymes breakdown cellulose into glucose which isquickly fermented into ethanol.

.GLU('4...,I;)Q)~- • 'e'.1M "'

CELLULOSE CHAIN~ o,u\.UC4",-?

SHORTER~fJ~CELLULOSE CHAIN

u\.uc0.r. ~'!f(])~:;; . ~ca •••

CILLOBIOSEFRAGMENTS

~

YEAST + GLUCOSE~

ETHANOL

be higher after removal of the carbo-hydrates. As a result, revenues fromprotein sales could remain the same ifthe product is valued by its proteincontent while shipping costs of thecorn gluten feed could drop by asmuch as 42%.

Utilization of theCarbohydrates inDDG5 for the Dry MillingProcessThe remaining ethanol made in

the U.S. is from the dry milling ofcorn58iY In this process, 2.5-2.6 gal ofethanol are produced per bushel ofcorn processed, along with about 17Ib of an animal feed coproduct calledDDGS, and 17 lb of carbon dioxide.DOGS contain" at least 27% protein.Essentially. all the cellulose .ind hemi-cellulose [rom corn ends up in theDDGS product, along with somestarch. The price ofDDGS seems to hescr largely hy its protein content, giv-

MARCH/APRil 1993

ing little value to the cellulose aridhemicellulose. In fact. these c.irbohy-drares add to shipping costs. As forwet milling. a process that convertsthe cellulose and hemicellulose into avaluable product such as ethanolcould enhance revenues for the plantwhile lowering shipping costs forDDGS.

Because the cellulose andhemicellulose comprise 9.5% of thedry weight of corn, -1.3 lb of these ma-terials are expected in 17 lb of DDGSfrom one bushel of corn. If convertedinto ethanol, the hemicellulose andcellulose in DDGS could yield 0.37 galof ethanol from every bushel of cornprocessed. This is an increase of 14-15% over the ethanol typically pro-duced from a bushel of corn. Sale ofthis increased ethanol would raise rev-enues similarly, while possibly main-taining animal feed coproduct rev-enues as a higher value protein feedand decreasing shipping costs by asmuch as 25%. For every 100 millionbushels of corn processed, totalethanol production from the hemicel-lulose and cellulose content of DDGScould be 37 million gallons.

ConclusionsEthanol is a clean-burning,

high-octane fuel that can be used intoday's transportation sector to im-prove air quality. Cellulosic biomassis an abundant resource in the UnitedStates that could support large-scaleproduction of ethanol for fuel use.This means that using ethanol derivedfrom biomass could reduce the strate-gic vulnerability of the United States,prevent sudden changes in price, andlower our trade deficit.

Both acid- and enzyme-cat-alyzed reactions have been evaluatedfor conversion of cellulosic biomassinto ethanol, and research has beenfocused on enzymatic hydrolysis tech-nology because of its potential toachieve high yields of ethanol undermild conditions. In particular, the SSFprocess is favored for ethanol produc-tion from the major cellulose fractionof the feedstock because of its lowcost potential. Technology has alsoheen developed for converting thesecond-largest fraction, hemicellulose.into ethanol, and the remaining lignincan be burned as boiler fuel to power

the conversion process arid generateextra electricity for export. Together.developments in conversion technol-ogy have reduced the selling price ofethanol from about 53.60/gal ten yearsago to $ 1.27/gal now. Additional tech-nical targets have been identified thatcould bring the selling price clown to

50.67/gal with aggressive research anddevelopment.

Technology for conversion ofcellulosic biomass to ethanol could beapplied to both wet and dry millingprocesses to su bstantially enhanceethanol revenues, while likely main-taining revenues from the sale of ani-mal feed coproducts with higher pro-tein content. Because the carbo-hydrates in these products have lowvalue, the application of cellulose con-version technology to these processescould result in lower ethanol costs forthe plant. Other low-cost materialscould also be converted into ethanolat prices competitive in today's fuelmarket.

References1. U.S. Department of Energy. May1990. Annual Energy Review 1989.Energy Information Administration.Washington, DC: Government PrintingOffice.2. Wald, M.L. 1990. "Greater Relianceon Foreign Oil Feared as U.S. OutputTumbles." New York Times, January18.3. U.S. Environmental ProtectionAgency. 1987. Air Quality Benefits ofAlternative Fuels. Washington, DC:Office of Mobile Sources.4. Browning, L.H., Pefley, R.K., andSuga, T.P. 1981. Proceedings of the16th Intersociety Energy ConversionEngineering Conference, Atlanta, GA.August 9-14.5. U.S. Department of Energy. 1978.Comparative Automotive Engine Op-eration When Fueled with Ethanol andMethanol. HCP/W1737-01 UC-96.Washington, DC.6. Wilson, R.D. 1987. Statement Be-fore the Consumer Subcommittee onCommerce, Science, and Transporta-tion, U.S. Senate. November 12.7. \'(lyman, C.E., a nd Hinman, N.D.[990. Applied Biochemical Biotech-nology. 24/25:7358. Lynd, L.R. 1990. Applied Biochem-ical Biotechnology. 24/25: 695.

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9. Lynd. L.R., Cushman, ].H., Nichols,R.J., and Wyman, C.E. 1991. Science251:1318.10. Lynd, LR. 1989 Advances in Bio-chemical Engineering. 38:1.11. Wenzl, H.F. 1970. The ChemicalTechnology of Wood. New York: Aca-demic Press.12. Wright, ].D. April 1983. High Tem-perature Acid Hydrolysis of Cellulosefor Alcohol Fuel Production. SERI/TR-231-1714. Golden, CO. Solar EnergyResearch Institute.13. Wright,J.D. 1988. Energy Progress8(2):71.14. Wright, J.D., and Power, A.].1985a Biotechnology BioengineeringSymposium. 15:511.15. Wright, J.D., Power, A.]., and Berg-eron, P.W. June 1985b. Evaluation ofConcentrated Halogen Acid HydrolysisProcesses for Alcohol Fuel Production.SERI/TR-232-2386. Golden, CO. So-lar Energy Research Institute.16. Mandels, M.L., Hontz, L., and Nys-trom,]. 1974. Biotechnology and Bio-engineering. 16:1471.17. Wilke, C.R., Yang, R.D., andStockar, U.V. 1976. Biotechnologyand Bioengineering Symposium Series.6:155.18. Gauss, W.F., Suzuki, S., andTagaki, M. 1976. "Manufacture of Al-cohol from Cellulosic Materials UsingPlural Ferments." U.S. Patent3,990,944, November 9.19. Tagaki, M., Abc, S., Suzuki,S.,Evert, G.H., and Yata, N. 1977.Biotechnology Symposium, 551.Delhi, India.20. Wright, J.D., Wyman, C.E., andGrohman, K. 1988. Applied Bio-chemical Biotechnology. 18:75.21. Veldhuis, M.K., Christensen, L.M.,and Fulmer, E.!. 1936. Industrial En-gineering Chemistry. 28:430.22. Ng, T.K., Weimer, P.J., and Zeikus,].G. 1977. Applied Microbiology.114:1.23. Cooney, c.i., Wang, D.LC., WandS.D., Gordon, ]., and Jiminez, M.1978. Biotechnology BioengineeringSymposium Series. 8:103.24. Wright, J.D. August 1988. Chemi-cal Engineering Progress 84(8);62-74.25. Target, R., Werdene, P., Himmel,M., and Grohmann, K. 1990. AppliedBiocherrucal BiorechnoJogy.24/25: 115.26. Wright,J.D. April 1983. High Tem-perature Acid Hydrolysis of Cellulose

74 FUEl REfORMUlATION

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MARCH/APRil 1993

Ethanol sellingprice ($/gal)

1.30~--------------'

1.20

1.10

1.27

13

50~''''7

Figure 4. The current price to ethanol from cellulosic biomass is $1.27/gal when the cost of thefeedstock is S42/ton. Lower cost feedstocks result in a lower cost of ethanol production. as shownhere.

1.00

0.90

0.80

0.70

0.60L...-------------~-10 o 10 20 30 40

Cost of biomass ($/dry ton)

Price ($/ton)

250~------------------------------~Corn gluten meal 'I: 7

200~Soybean meal D.

1501-ODGSD

1001-Corn gluten feed 0

60~ ...

Figure 5. Relationship among the market prices of various animal feeds and their protein content.based on recently reported prices.

501-

O~--------_I~--------~I-------~o 20 40Protein content (%)

ET170: 7/22/92