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Alcohol production from woodDavid L Brink
Ligno ce l lu lose - th e mater ial fo rming thewoo dy cell walls of plants-represents th esingle largest supply of polysaccharidescarbohydrates) produced in the plant king-
dom that can be hydrolyzed to sugars andconverted in to fuel a lcohol . Biomass mate-rials tha t are prepon derantly lignocellulosicinclude all woo d residues generated in log-ging and sawmilling oper ations; pru nings oforchard, vineyard, and ornamental plants ;stalks of co tton pla nts; and stems of grassesincluding whea t, rice, barley, co rn stover),sugarcane bagasse after extraction ofsu-crose), and b amb oo.
Because of cell wall and molecular stru c-tures, the polysaccharides, and especiallythe cellulose, of lignocellulosic materials arerelatively inaccessible to enzymatic hydroly-sis, when compared with other materialssuch as starch, an d they req uire severe con-ditions in acid hydrolysis. However, eitherdilute acid or concentrated acid hydrolysisof these materials is technically feasib le o nacommercial scale.
Designs of processes using either type ofacid hydrolysis have disadvantages, andboth involve complicated technology. In
any hydrolytic p rocess using lignocellulosicmater ials to produce ethan ol , i t will be es-sential to comp letely utilize th e raw materi-al, including the 25 to 40 percent insolubleligneous residue produced in hydrolysis; tobe energy efficient; and to be environmen-tally benign.
The process also should be able to ac-commodate a wide spectrum of biomass,including materials with a low polysaccha-r ide content , such as bark of t rees andshrubs, or little or no l ignin, such as herba-ceous plants, leaves, fruit pulp and otherresidues from food-processing plants, andspoiled or surplus fruits, vegetables, andgrains. Biomass materials received at aplant site will have moisture co ntents vary -ing from virtually overdried t o over9 er-cent green basis). N o processing stepshould be needed to adjust moisture con-tent except, perhaps, b lending different rawmaterials to provide a more uniform feed-stock. Drying should be avoided exceptunder the unusual c ircumstance that mois-ture content is so high that i t would beeconomically advantageous.
16 C A L I F O R N I AAGRICULTURE,JUNE 1980
SolidAc ld (wood) Wate r Ai r
Stage Ihydrolysis
Sensi t izat ion
OP
Stagehydrolysis
C = separa to r
SolventAddit ives (make-up) Products
L iqu idextract ion
NO. 2
a
OP
Excess yeast(cattle feed)
Ethanol
Methane
Acetic acFormic ac
Methanour fu ra_ _. ntN O . 1
_ _Excess ste
Feed water
Lignocellulosic plant materials can be convertedto alcohol by th e oxidative hydrolysis-wet oxidation-fermentat ion pr oce ss shown in this schem atic .
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The new process operation. Basically, the system consists ofA process designed at the University of hydrolysis in steps to produce the solutions
California Forest Products Laboratory of monosaccharides simple sugars); fer-meets the criteria set for th above and is be- mentation of the sugar solutions to produceing studied to develop specifications for ethanol or other products; methanation toequipment and to perfect it for full-scale convert to methane the soluble organic pro-
Wood and other lignocellulosic materialsare abundant sou rce s of carbohydrates forconversion to alcohol, but their cell wallsand molecular structures make them diffi-LIGNO.
CELLULOSIC cult to break down.MATERIAL
Product yields per ovemdried ton of lignocellulosic material
Whit e Cotto n Sugarcane Cerealfir Maple stalks bagasse grain Rice
p o u n d s gallons in parentheses)
WET OXIOATION
ETHYL ALCOHOL
METHYL ALCOHOL
METHANE
THERMAL ENERGY
Broduc? yields from white f ir.
37657)
56(9)
243
42765)
447)
211
To process)
37056)
315)
193
33851)
233)
160
ORGANIC ACIDS t OTHER ORGANICS,
ORATERIAL U G A R S
+Ffl-HTkFlATION LATION NATION DATION
LIGNEOUS RESLDUE
mWHITE LIGNEOUS ETHYL METHYL
IR SUGARS RESIDUE ALCOHOL ALCOHOL METHANE THERMAL TOTALPounds 2,000 1.422 522 376 56 243 675
3.19 14.33IU X lo 16.96 9.57 6.49 4.82 0.54 5.78
Percent (wt) loo 71.1 26.1 18.8 2.8 12.1 33 7
Percent (Btu) 100 56.4 38.3 28.4 3.2 34.1 18.8 84.5
ducts that are not isolated from the watersolutions; and wet oxidation to convertsolids produced in the process to soluble or-ganic products that can be methanated orisolated, carbon dioxide, and thermalenergy. This process is illustrated by theschematic diagram. The process flow showndoes not present the simplest design, but itillustrates the potentially available products
using a relatively basic design. This designcan be expanded to produce a greater vari-ety of products.
The raw material is prepared by hammer-milling, using a minimum of energy to pro-vide the optimum particle size. Afterremoval of tramp metal, sand, and the like,the hammermilled material is introduced in-to the stage I pressure vessel at the as-received moisture content.
Stage I hydrolysis removes accessiblepolysaccharides, largely hemicellulose,from the feedstock. The result is a particlethat can be readily crushed or refined to
provide finely divided particles in a slurry.Substantially less energy is used than wouldbe required to reduce the feedstock to thesame size by only mechanical means.
Sensitization and stage I1 hydrolysishydrolyze the more resistant crystalline) orinaccessible cellulose and polysaccharides.Sensitization with oxygen increases the rateof cellulose hydrolysis. An acid added inthis system provides a pH that will give anacceptable rate of hydrolysis at the elevatedtemperature used. Thus, this process is ofthe dilute acid type. The state I1 product-the hydrolyzate-may be introduced to
stage I to carry out the hydrolysis of the ac-cessible polysaccharides. Or separatehydrolyzates may be removed from eitherstage. If the latter design is used, acid and astream of water from methanation mustalso be introduced to stage I.
Liquid extraction number may be re-quired to remove any organic product fromthe hydrolyzate that may decrease the ratesor the yields obtained in the fermenta-tion reaction. Also, it is an optional stepthat can be taken to isolate extractableorganic products, if such products are pro-duced in sufficient amounts to make the ad-
ditional processing economically attractive.Fermentation is carried out using conven-
tional equipment and techniques. This willinclude the modification that will be neces-sary to ferment pentoses 5-carbon sugars)when this technology is developed. Pen-toses comprise about 2 to 30 percent, andhexoses 6-carbon sugars like glucose)about 45 to 6 percent of the weight of lig-nocellulosic materials.
Rectification of the beer from fermenta-tion to produce ethanol at the desired purity
C A L I F O R N I A GRICULTURE, U N E 1980 17
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an d other volatile produ cts fuse1 oil) is per-formed using conventional equipment andprocessing.
Methanation produces methane fromproducts in the aqueous still bottoms dis-charged from rectification and in theaqueous system discharged from wet oxida-tion. Again, conventional procedures andequipment are used.
Most of the water discharge from m etha-nation is recycled to wash the solids inC-2and C-1 and some is discharged as wastewater from the process. The ratio of washwater to waste water determines the buildupof soluble inorganic m aterials in the processwater. Make-up water not shown in theschematic) is added to C-1 to improvewashing of soluble hydrolysis productsfrom the ligneous residue.
The am ount of m ake-up water adde d iscontrolled to m aintain the water balance inthe system. T hus, m ake-up w ater is the dif-ference between water outp ut an d water in-put excluding make-up water). Th e outp utcomprises water in the waste water dis-charge and water vapor in the off-gases ofwet oxidation and fermentation and in theproduce gas from methanation. The input ,excluding make-up water, is water chargedto the system in the raw material, in the acidreagent, and in the solution of additivesused in fermentation.
The soluble solids content of the waste-water discharge is essentially inorganic, be-cause soluble organics are consum ed in themethanation step. This inorganic materialoriginates from the soluble inorganic com-pounds present in the feedstock and fromsmall amounts of chemicals and nutrientsadde d during processing. T hese materials inthe waste water should function as soilamendm ents when that water is used fo r ir-rigation.
Wet oxidation of the washed ligneousresidue and organic solids from fermenta-tion and methanation produces a largeamount of heat used to produce steam.Either oxygen in air or gaseous oxygen isused as the oxidant in this step. Dependingon conditions used, biomass is oxidized tovariable amounts of products includingwater, carbon dioxide, a small amount ofcarbon monoxide, a mixture of organicacids, neutrals, and finely divided, partiallyoxidized, ligneous particles. Ultimateproducts of wet oxidation are carbondioxide and water.
Wet oxidation can be carried out to maxi-mize production of m ethane o r to maximizeproduction of thermal energy. It is highlypertinent that this thermal energy suppliesall of the requirements of the process.Thu s, the process is energy self-sufficient.
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The off-gases from wet oxidation are ex-panded through turbines to produce energyand are then processed to remove the smallamount of carbon monoxide and traceamounts of any volatile organics that maybe present. The volum e of the gas to be pro-cessed will be greatly reduced if gaseousoxygen rather than air is used in wet oxida-tion. Th is would simplify any steps required
for removing trace amounts of organiccompounds that would be present in theoff-gas from the process.
Liquid extraction num ber is an optionalstep that ca n be included to isolate organicacids and some neutral products . Som e ofthe compounds that can be produced aregiven in th e schematic.
Potential 01 Alcohol and Energy lrom Lignocellulose asan Alternative to Motor Fuel n the United States
Fuel P rD du c li on ~ q i ~ ~ 1 ~ n l k C r e l y e a r 'gal s lU
Ethanol 580 4.894 x 107
Methanol 70 o 444 107
Importitem equivalen t Total
54.5 1156 8 14 26
Acreage lo replace motor fuel Withethanol 8 methanol (x 106 1266 267 1
1 forest and cropland 10 7 22 6
Methane produced. Blu x 6 76 14 27
Total energy produced. Btu x 13 52 28 53
Produced by hydrolys81-wef oxidalion-lermenlsliOn. assuming 10 tons Oven drled hgO C e l l U l o S l C matsr la l per acre oer yea r?Foresf land and cropland 1,161 x 1 6 acres. F o r e r l S l a l i s t ~ c s or the Uniled SlalesF o r es t S e r u i c er. U S Department 01 A g r ~ c ~ I l u r e .972. P 2
Of IDIal energy COnsumpIlOn. 1977 17 6 37
Conclus ionTh e new process, designed to fully utilize
the raw m aterial, to be energy-efficient, andto be environmentally benign, also accom-modates a wide variety of biomass m aterialsand an extreme range of moisture contents.It produces only three products-ethanol,methane, and thermal energy-but has theflexibility to pro duce additio nal m aterials-yeast that may be used in food or feed,organic acids, and several neutral com-pounds.
Th e system can produce one hydrolyzatecontaining all monosaccharides or, whenprocessing biomass from angiosperms,separate hydrolyzates, one w itha high per-centage of glucose an d the oth er with a highpercentage of xylose a pentose). Th e bio-logical agents used in fermentation can bevaried to provide a variety of metabolicproducts.
The thermal energy that ca n be developedby wet oxidation can be used to supply heatto other steps of the process so that it isenergy-independent. T he process has a lowwater requirement because of its high ratioof recycled water to discharged water. Thewaste-water discharge contains the inorgan-
ic matter of the biomass raw materials withsmall increments o f inorganic agents addedto control hydrolysis and fermentationsteps. Organic matter is essentially absent.
Alcohol production from lignocellulosecan provide a substantial percentage of theenergy used in the United States, and thiscould be accomplished as rapidly as anyprogram involving utilization of fossil or
nuclear fuels. By processing of the type de-scribed, the amount of alcohols and meth-ane produced from a ton of representativefeedstocks is given in the table of processsteps and products. The yields obtainedfrom white fir as well as energy contents ofthese fuels are given in the table fo r white fir.Using th e latte r as typ ical of lignocellulose,in general, the potential of alcohol produc-tion by processing of this kind is given inthe table showing the imp ort equivalent andtotal motor fuel consumed in the UnitedStates in 1977.
For illustrative purposes, the basic as-sumption is ma de that 10 ton s of lignocellu-losic residue is produced annually per acre.Then the amo unt of alcohols produced on126.6 million ac res of land would be equiva-lent to the mo tor fuel produced from crudeoil imported in 1977. This amounts to 6.76x 1015 = quadrillion) Btu or 6.76quads of energy. The total energy used inthe United S tates during 1977 amounted toabout 77 quads. An amount of energyalmost equivalent to that in the alcohols isproduced as methane. Thus, the total pro-duction of energy from the alcohols plusmethane would have amounted to about17.6 percent of the national energy con-sumption of that year. To replace the totalmotor fuel consumed that year, 267.1 mil-lion acres would be required. These landareas of 126.6 and 267.1 million acresam ount to approximately10.7 and 22.6 per-cent of the tota l U.S. forest and croplands.It is to be emphasized that residues, no t theprimary p rodu ct, w ould be utilized.
In addition t o weaning the United Statesfrom petroleum-based fuel,a national pro-gram to utilize lignocellulose in productionof this kind w ould avoid the potential haz-ards of nuclear energy and the environmental impacts of coal utilization. Indeed, sub-stantial benefits would accrue due to in-creased productivity from our crop andforest lands, increased employment opportunities, a new and stable all-year industry,and improvement in, rather than degrada-t ion of , o ur environment.
David L . Brink is ProJessor Forest Prod ucts Labora-tory Universify OJ California Richmo nd. The authoracknowledges the advice and support of GeoProductsCorpo ration of Oak land California. in this research.
