APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Jan. 1986, p. 115-1220099-2240/86/010115-08$02.00/0Copyright © 1986, American Society for Microbiology

Intermediate-Scale, Semicontinuous Solid-Phase FermentationProcess for Production of Fuel Ethanol from Sweet Sorghum

WILLIAM R. GIBBONS,' CARL A. WESTBY,L* AND THOMAS L. DOBBS2

Departments of Microbiologyl and Economics,2 Alcohol Fuel Research Laboratory, South Dakota State University,

Brookings, South Dakota 57007

Received 23 May 1985/Accepted 12 October 1985

A novel, semicontinuous solid-phase fermentation system was used to produce fuel ethanol from sweetsorghum. The process was at an intermediate scale. In the process, dried and shredded sweet sorghum was

rehydrated to 70% moisture, acidified to pH 2.0 to 3.0, and either pasteurized (12 h at 70 to 80°C) or not

pasteurized before spray inoculation with a broth culture of Saccharomyces cerevisiae. Fermented pulp exitedthe semicontinuous fermentor after a retention time of 72 h and contained approximately 6% (vol/vol) ethanol.Ethanol yields from dry sweet sorghum were 176 to 179 liters/103 kg (85% of theoretical). Production costs fora greatly scaled-up (x 1,400) conceptual version of this system were projected by calculation to average

$0.47/liter for 95% ethanol. The calculated energy balance (energy output/energy input ratio) was estimated tobe 1.05 when pasteurization was included and 1.31 when pasteurization was omitted. In calculating the energy

balances, the output energy of the protein feed byproduct and the input energy for growing the sweet sorghumwere not considered. A design for the scaled-up plant (farm scale) is provided.

Sweet sorghum [(Sorghum bicolor (L.) Moench)] has thepotential of becoming a useful energy crop. It has beenevaluated in several recent reports as an alcohol fuel crop

with a promising future (12, 14). The primary advantages ofsweet sorghum are: (i) its high ethanol productivity, 3,700 to5,600 liter/ha per year (14); (ii) its adaptability to diverseclimatic and soil conditions (12, 14); and (iii) its reduced needfor nitrogen fertilizer (14) and water (12, 14) when comparedwith more conventional crops such as corn.

Despite these advantages and the predictions, sweet sor-ghum has yet to become a viable alternative to corn in fuelethanol production. The major reasons for this are that mostproposed processing strategies, which are based on separat-ing and then fermenting the sugar fraction of the stalk, arecurrently either uneconomical, energy inefficient, or un-proven on a commercial scale.

Processes that have been so examined include: (i) mechan-ically expressing juice from whole sweet sorghum stalks atharvest and fermenting the juice directly or after storage as

a concentrate (K. Q. Stephe.son, Summer Meet. Am. Soc.Agric. Eng. 1983, 83-3064); (ii) using a Tilby separator toremove pith from 10- to 20-cm stalk billets and fermentingjuice pressed from the pith (12, 13, 19, 23); and (iii) using theEX-FERM process, in which sugar from small chips isextracted and fermented simultaneously in an aqueous solu-tion (2, 17-20).

Recently Bryan and Parrish (W. L. Bryan and R. L.Parrish, Winter Meet. Am. Soc. Agric. Eng. 1982, 82-3603)demonstrated in laboratory-scale tests with flasks that solid-phase fermentation could be used to produce ethanol fromchopped sweet sorghum. Kargi and Curme (8) observed thesame sort of result with sorghum pith in a laboratory-scalerotary-drum fermentor. We found in farm-scale runs thatsolid-phase fermentation could also be used for ethanolproduction from ground fodder beets (6). The advantages ofusing solid-phase fermentation rather than submerged fer-mentation of sweet sorghum (10) are: (i) greater fermentorproductivity (ethanol production per unit volume), (ii) reduc-

* Corresponding author.

tion in required fermentation capacity, (iii) reduced need fornutrient addition, (iv) lower production costs, (v) lowervolumes of stillage for disposal, and (vi) less energy fordistillation. We present microbiological and economic find-ings here on intermediate-scale conversion of sweet sorghumto fuel ethanol by a semicontinuous, solid-phase fermenta-tion process.

MATERIALS AND METHODS

Sweet sorghum harvesting and storage. Standing sweetsorghum [(Sorghum bicolor (L.) Moench, var. PAG morcane)] was cut approximately 5 cm above ground level byusing a side-arm mower. Stalks were then tied into loosebundles and transported immediately to the alcohol plant.The bundles were subsequently dried to 15% moisture in a

48°C forced-air dryer (14 days). Dried stalks were thenshredded by passage through a tub grinder (screen size, 2.54cm). The shredded material (15% moisture) was bagged(plastic) and stored at room temperature until use.Inoculum preparation. Saccharomyces cerevisiae NRRL

Y-2034 was maintained on malt agar (Difco Laboratories,Detroit, Mich.). Three serial transfers of Y-2034 culturesinto progressively larger vessels of the inoculum mediumwere used to build up the yeast population over a 2-day spanfrom a few million cells in 100 ml to a final inoculum ofapproximately 3 x 1011 cells in 3 liters (108 cells per ml). Theinoculum medium contained 4% glucose and 0.5% each ofneopeptone (Difco), yeast extract, and malt extract. Allyeast cultures were grown at 30°C without agitation.

Semicontinuous, solid-phase fermentation. The semi-continuous, solid-phase fermentation device used in ourfarm-scale plant is shown in Fig. 1. It was constructedentirely of mild steel and consisted of: (i) a nonported steampasteurization chamber to destroy bacterial contaminants inthe shredded sweet sorghum, (ii) a yeast inoculation port,and (iii) an auger that simultaneously conveyed and mixedthe fermenting pulp. The auger could be either turnedmanually, as was the case in our experiments, or rotatedwith a slow-speed motor.

Before each new run, the pasteurization chamber and

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116 GIBBONS ET AL.

ShreddedSweet Sorghum

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FIG. 1. Intermediate-scale, semicontinuous, solid-phase fermentor for sweet sorghum and other high-moisture, high-fiber feedstocks.

auger of the fermentor were steamed for 12 to 18 h to killcontaminating microorganisms that might have been leftover from the previous run. Dried, shredded sweet sorghum(15% moisture) was rehydrated and acidified to pH 2.0 to 3.0by mixing together 0.5 kg of dry sweet sorghum, 1 liter ofH20, and 10 ml of 36 N H2SO4. The acidified pulp (1.51 kg at70% moisture) was then placed into the pasteurization cham-ber for 12 h of pasteurization at 70 to 80°C.

After pasteurization, portions (0.45 to 0.55 kg) of the pulpwere dropped into the front end of the tubular fermentor byremoval of a sliding partition. The pulpy mass was thenallowed to cool to 30 to 40°C (5 to 10 min) before beingthoroughly spray inoculated with a broth culture of S.cerevisiae NRRL Y-2034. The spray inoculum contained 108cells per ml, and 0.1 ml was used for each gram of wet pulp(massive inoculum was used to minimize the fermentationtime). Access for inoculation was through a port on thefermentor. After inoculation, the auger (Fig. 1) was manuallyrotated 3600 two to three times. The inoculation and augerrotation steps were then repeated twice to empty the pas-teurization chamber, whereupon the chamber was refilledand pasteurization was redone. The auger was large enoughto accomodate seven batches of acidified and inoculatedpulp (1.66 kg per batch) at any one time. In some trials, thepasteurization step was omitted. Here, quantities (0.45 to0.55 kg) of the acidified pulp were dropped directly into thefront end of the fermentor and were inoculated immediatelyas described above.With or without pasteurization, the above procedure (ex-

cept for steaming the auger) was repeated at 12-h intervalsfor up to 400 h. Due to the length of the auger and its slowrate of rotation, entering pulp did not exit from the fermentorfor 72 h, permitting sufficient time for complete fermenta-tion. The calculated efflux rate from 72 h to the end of a

typical fermentation run was 0.12 to 0.13 kg of fermentedpulp per h.Samples of raw, inoculated, and fermented sorghum pulp

were collected at 12-h intervals and were measured for

reducing sugar (6, 16), ethanol (7), yeast and bacterialpopulations (22), moisture (7), crude protein (7), crude fiber(7), ether extract (7), and ash (7). The pulp pH in differentparts of the fermentor was determined by rapidly augeringout the contents into fractions and measuring the pH in eachof the fractions.

RESULTS AND DISCUSSIONSweet sorghum analysis. The composition (wt/wt) of fresh

sweet sorghum stalks used in this study was as follows. Themoisture content averaged 70%, while reducing sugar con-tent was 10.4%. Approximately 75% of this sugar wassucrose (measured by the DNS method [16] after hydrolysisby invertase); the remainder consisted primarily of glucoseand fructose (12). The remaining components were as fol-lows: ash, 2.32%; crude protein, 1.89% (mostly protein inamine or ammonium nitrogen form); ether extract, 0.73%(mostly fat; all volatile substances were removed beforeextraction); and crude fiber, 6.96%. Assuming a conversionrate of 53.8 g of ethanol per 100 g of reducing sugar (W. L.Bryan, G. E. Monroe, R. L. Nichols, and G. J. Gascho,Winter Meet. Am. Soc. Agric. Eng. 1981, 81-3571), themaximum theoretical ethanol concentration in fermentedpulp from this sweet sorghum would be 7.09% (vol/vol).

Conventional submerged fermentation. Preliminary testswith conventional submerged fermentation of shreddedsweet sorghum showed this process to be too costly andenergy intensive to warrant further study. The energy bal-ance was less than 1.0, and the cost was $0.53 to $0.66/liter($2.00 to $2.50/gal), assuming a sweet sorghum cost of$51.16/103 kg ($46.40/ton). These findings are similar to thosewe previously noted for fodder beets (6).

Semicontinuous, solid-phase fermentation. By using thesemicontinuous, solid-phase fermentor shown in Fig. 1, weinvestigated two different operational modes for producingfuel ethanol from sweet sorghum. In the first operationalmode, dried and ground sweet sorghum (15% moisture) wasrehydrated, acidified, and pasteurized before yeast inocula-

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Fermentation Time (h)FIG. 2. Ethanol production from acidified and pasteurized sweet sorghum pulp with an intermediate-scale, semicontinuous, solid-phase

fermentor. The sweet sorghum pulp was adjusted to pH 2.0 to 3.0 and pasteurized before inoculation. The efflux rate was 0.12 to 0.13 kg offermented pulp per h, the exit pH was 3 to 4, and the retention time for each batch was 72 h. Levels of reducing sugar and ethanol and theyeast population were measured in the exiting pulp. Symbols: *, reducing sugar; A, ethanol; *, yeast population.

tion. Pasteurization was for 12 h because of the poor heatflow characteristics of the beet pulp and because 12-hperiods were convenient for repeating the process over thelong duration of the run. Pasteurization and acidificationwere both used since we had previously found that these twoprocedures, when used together, eliminated contaminationproblems in fodder beet pulp (6). The acidification stepreduced the pH to 2 to 3. Such a drastic pH reduction wasrequired because the mild steel parts of the fermentorcontinually increased the pulp pH during fermentation to 3 to4, and this somewhat higher pH level was just barely capableof controlling contaminants. When the initial pulp pH was 3to 4, the pH during fermentation rose to a level that wasunacceptable for contamination control. If a stainless steelfermentor was used instead, as would likely be the case in acommercial plant, an initial pH of only 3 to 4 might suffice,and under these circumstances, two to three times lessH2SO4 would be required for pH adjustment.The results from a trial in which acidified and pasteurized

sweet sorghum pulp was used are shown in Fig. 2. A total of44.8 kb of pulp was processed through the fermentor duringa 400-h period. This amounted to 27 1.66-kg batches, eachwith a retention time of 72 h. The ethanol concentration inthis run increased in the fourth and succeeding batches from0% (vol/vol) at inoculation to 5.6 to 6.0% (vol/vol) at exit.This corresponds to an ethanol yield of 179 liters/103 kg ofdry sweet sorghum or approximately 85% of the theoreticalyield. The residual reducing sugar concentration in thefermented pulp-varied between 0.4 and 0.8% (wt/wt) com-pared with 10 to 10.5% in freshly inoculated pulp, and theyeast population increased in the fourth batch and thereonfrom 1 x 107 cells per ml at inoculation to 2.7 x 108 cells perml at the end of the 72-h retention time. Bacterial contami-nants were not detected (data not shown). A retention timelonger than 72 h (slower auger rotation) might have increased

the yeast population more and lowered the reducing sugarconcentration further; however, calculations showed thatthe rates would have been too slow to make this economi-cally feasible. In fact, a more extensive technical-economicanalysis showed that the retention time should have been 48to 54 h if the deciding factor was the lowest cost.These results are similar to those we found in a previous

study, in which acidified and pasteurized fodder beet pulpwas fermented in the solid-phase fermentor (6). In thatstudy, the ethanol concentration in the fermented fodderbeet pulp rose to 8.5 to 9.0% (vol/vol), while levels ofresidual reducing sugar and yeast cells remained constant at1% (wt/wt) and 2.5 x 108 cells per ml, respectively. Heretoo, bacterial contaminants were not detected in the fer-mented pulp.

In the second operational mode, sorghum pulp wasrehydrated and acidified but was not pasteurized beforeinoculation. This was done to see whether acidification alonewas sufficient for contamination control. Figure 3 showsresults from one such trial. In this trial, the ethanol concen-tration in the fermented pulp again rose to a level ofapproximately 6% (vol/vol), which corresponds to anethanol yield of 179 liters/103 kg, or 85% of the theoreticalyield. The residual reducing sugar concentration in the pulpremained below 0.6% (wt/wt), and the yeast cell populationrose to 5 x 108 cells per ml. Bacterial cells were detected inthe fermented pulp at levels up to 103 cells per ml (data notshown); however, such levels were also detected in freshlyinoculated pulp. The results indicate that, while the acidifi-cation process does not kill all types of contaminants, it doescreate conditions adverse to the growth of the survivors.Furthermore, ethanol yields were the same as when pasteur-ization and acidification were both used, obviating anyfuture need to pasteurize acidified sorghum pulp used forethanol production.

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FIG. 3. Ethanol production from acidified sweet sorghum with an intermediate-scale, semicontinuous, solid-phase fermentor. The sweetsorghum pulp was adjusted to pH 2.0 to 3.0 before yeast inoculation. Levels of reducing sugar and ethanol and the yeast population weremeasured in the exiting pulp. Conditions and symbols are as described in the legend to Fig. 2.

Three factors appear to have been primarily responsiblefor control of contaminants in the nonpasteurized, acidifiedpulp: low pH, anaerobiosis, and ethanol inhibition. The lowpH (2.5 to 3.5) in the first third of the fermentor was mostlikely the main cause for inhibiting bacterial contaminantsthere. In the remainder of the fermentor, where the pH wasnearer neutrality and the ethanol concentration was higher,the bacteria that survived acidification were most likelyinhibited by a high ethanol concentration and yeast-generated anaerobiosis rather than by low pH.

Recently, Bryan and Parrish (W. L. Bryan and R. L.Parrish, Winter Meet. Am. Soc. Agric. Eng. 1982, 82-3603)completed a small-scale batch study with shredded sweetsorghum (variety Wray) as the feedstock. Their results aresimilar to ours for semicontinuous processing at a muchscaled-up level. They obtained ethanol concentrations in thefermented pulp ranging from 7.6 to 8.2% (vol/vol) after 60 hof solid-phase fermentation. The ethanol yield was 78 to 80%of the theoretical value. They postulated that the relativelylow ethanol yields may have been caused by naturallyoccurring fermentation inhibitors in sweet sorghum varietyWray. This effect had previously been noted by Day andSarkar (1). We used a different sweet sorghum strain (PAGmor cane) and different growing conditions and observed nosuch inhibition, while obtaining, as noted, 6.0% (vol/vol)ethanol after 72 h of continuous fermentation.Bryan and Parrish had no serious contamination problems

in their 7-liter batch solid-phase fermentations, even thoughthe sweet sorghum was neither pasteurized nor acidifiedbefore inoculation. This may have been due to the largeyeast inoculum they used and the fact that they used batchfermentation. In a commercial-scale production system withfodder beets, we encountered severe contamination prob-lems after 4 to 6 days of continuous fermentation whenacidification and pasteurization were not part of the produc-tion process (6). On the other hand, with sweet sorghum,

acidification alone appeared to be effective in contaminationcontrol (Fig. 3). This has also been demonstrated by Kirbyand Mardon for sugar beets (10).At the end of the fermentation period (400 h) for the

second operational mode (Fig. 3), the fermentor contentswere rapidly (time span, 1.5 h) run out the end of thefermentor and analyzed immediately (no storage). The pur-pose was to obtain an instantaneous profile of the fermentorcontents as a function of fermentation time so as to be ablein the future to set as short a fermentation time as possible.The results (Figure 4) indicate that there was little differencein ethanol concentration of the pulp from one end of thefermentor to the other. This sort of result had not beenanticipated, and the main reason for it appears to have beenback mixing (pulp slippage and back-tumbling during augerrotation and diffusion throughout the run) which permittedan equilibration of the auger contents. Such mixing undoubt-edly was facilitated by the long residence time of the pulp (54to 72 h) and the fact that the auger was only loaded to 25 to30% of its capacity. We are presently testing shorter fermen-tation times (48 to 54 h) to establish more pronouncedethanol gradients. This should be useful in determining theexact fermentation time (within the 48- to 54-h span) to beused for maximum cost cutting.

Farm-scale alcohol plant design. The design and operationof a theoretical farm-scale plant to produce fuel ethanol anda pressed protein feed (PF) from sweet sorghum based uponthe research results reported herein are shown in Fig. 5. It isassumed that, during harvest, the majority of the sweetsorghum billets are dried to 15% moisture, at least in partwith waste heat from the alcohol plant, methane fromanaerobic digestion of waste stillage, solar-generated heat,or some other low-cost heat source. The dried billets arethen stored for use during the rest of the year. During theharvest period, however, a portion of the fresh billets isdiverted for direct processing to ethanol in the plant.

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fermentor does not include the time required for material to be augered out the opposite end at 400 h. This was 1.5 h for material at entranceof auger. Symbols are described in the legend to Fig. 2.

In either case, fresh or dried sweet sorghum billets are

conveyed to an automatic scale and dropped into a ham-mermill, where they are shredded into a pulp. Pulp from thehammermill is then adjusted to pH 3.0 to 4.0 or it isrehydrated and the pH is adjusted, depending upon whetherfresh or dried billets are used. The pulp is then released intothe fermentation section of the stainless steel auger, where itis inoculated with a spray of yeast cells. The first section ofthe auger could easily be modified to permit pasteurization(70 to 80°C for 3 to 12 h) of the acidified pulp, if necessary.In this instance, acidified and steamed pulp would first becooled before being augered further down the fermentor forsubsequent inoculation with a spray of yeast cells.

After inoculation, the pulp ferments as it is slowly augeredtoward the delivery end of the fermentor. Fermentationtemperature (28 to 32°C) is maintained by means of theheating-cooling jacket which encases the fermentor (Fig. 5).After 48 to 54 h of retention time, the fermented pulp dropsfrom the auger and is conveyed through two sets of rollermills, with an interstage wash (2, 4, 10, 11). This separatesthe pressed juice (beer) from the PF. The beer is subse-quently distilled to 95% (vol/vol) ethanol. Our design avoidspumping or direct column distillation of sorghum pulp be-cause the material is too thick and fibrous for these steps.Instead, as indicated, pressing of the pulp and subsequentdistillation of the beer is proposed. These have to be closelycoordinated so that one does not run too far ahead of or

behind the other. The rates for this coordination in our

farm-scale plant are 1,339 kg of pulp pressed per h and 83liters of 95% ethanol distilled per h (21).

Part of the stillage from the distillation of sorghum press-ings is, in our design, reused as yeast propagation mediumand pulp rehydration water. Pasteurization of the stillage inthe distillation column (18) and subsequent acidificationwould prevent contaminants from interfering with solid-

phase fermentation during such recycling. The rest of thestillage is disposed of as waste or is anaerobically digested toprovide methane for forced-air drying of the sweet sorghumbillets.

Farm-scale alcohol plant parameters. Table 1 lists theannual raw material requirements, the capacity, and the rateof production for a theoretical farm-scale plant producingfuel ethanol and a pressed PF from sweet sorghum by usingthe semicontinuous, solid-phase fermentation processshown in Fig. 5. An ethanol yield of 179 liters/103 kg of 15%moisture sweet sorghum is assumed.

In 1985, an estimated 0.7 x 106 ha of U.S. cropland was

planted to grow sweet sorghum (R. Christensen, personalcommunication). Assuming an average sweet sorghum yieldof 44 x i03 kg/ha (13, 15), the total United States harvest forthat year should have been 30.8 x 109 kg. If all of this hadbeen processed for ethanol, approximately 1.36 x 109 literswould have been produced, assuming an ethanol yield of 44liters/103 kg of whole sorghum plants (9); To process such a

quantity of sweet sorghum by using the farm-scale plantdesign described in this article, 2,281 such plants would havebeen required. If, instead, community-scale plants (output, 4x 106 to 8 x 106 liters/year) were available and used, only170 to 340 plants would have been required.Energy balance. The energy inputs, energy outputs, and

energy balances (energy output/energy input ratios) for a

theoretical solid-phase fermentation plant producing fuelethanol and PF from sweet sorghum with and withoutpasteurization are given in Table 2. In calculating the energybalances, the output energy of the PF and the input energyfor growing were not considered. The nonpasteurizationmethod has a significantly higher energy balance whencompared to the pasteurization method. This is due toelimination of the energy-intensive pasteurization process.

Costs. Cost estimates for the theoretical ethanol plant (Fig.

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FIG. 5. Design and operation of a theoretical farm-scale ethanol plant for conversion of sweet sorghum and other high-fiber crops toethanol and protein feed. Fermentor volume is calculated on the basis of a 48- to 54-h retention time with each kilogram of pulp occupyinga volume of approximately 1 liter and the fermentor operating at 75% of full capacity. Yeast cells are propagated under continuous cultivationconditions with a retention time of 3 to 4 h.

5) operated with sweet sorghum are shown in Table 3.Capital, operating, and other fixed costs are provided bothfor the pasteurization and acidification mode of operationand for the nonpasteurization and acidification mode. Totalcosts before and after deducting a credit for the PF by-prod-uct are shown. Costs for drying and storing the sweetsorghum are not included.Costs were estimated by comparing the theoretical plant

(Fig. 5) with a similarly sized corn-based ethanol plant,economically characterized previously by Dobbs and co-workers (3; R. Hoffman and T. L. Dobbs, SDSU Ag. Exp.Sta. Bull. 686, 1982). Whenever identical equipment orsupplies were used in each process, the data of Dobbs andco-workers were used directly as costs for the sweet sor-ghum-based plant. When input components were not identi-cal, the costs of Dobbs and co-workers were modified andthen used in estimating costs for the sweet sorghum plant.This same procedure was used in making previous costestimates for fodder beet-based ethanol production (6).

Preliminary estimates of costs (net of the PF credit) forethanol produced from sweet sorghum in a farm-scale plantare approximately $0.47/liter ($1.78 to $1.80/gal) (Table 3).This can be compared with our estimates for ethanol pro-

duction from corn and fodder beets (3, 5, 6, 22). Ethanolderived from corn by using our baseline method of plantoperation costs $0.47/liter ($1.78/gal) when corn costs are

$2.50 per bushel ($2.50/25.2 kg), and other prices are at 1981levels. When fodder beets are the feedstock, the ethanol isestimated to cost $0.46 to $0.47/liter ($1.74 to $1.77/gal).Thus, our preliminary estimates for producing ethanol fromsweet sorghum are very close to the previous estimates forethanol from corn and fodder beets.Costs of the sweet sorghum feedstock ($0.29/liter

[$1.10/gal]; Table 3) are higher than the baseline costs forcorn and fodder beet feedstocks. The sweet sorghumfeedstock cost was derived from the average of severalgrowing-cost estimates and assumed an ethanol yield of 179liters/103 kg (42.2 gal/ton) of dry (15% moisture) sweetsorghum. Five different growth cost estimates cited in vari-ous studies (12), (D. R. Jackson and M. F. Arthur, GasoholUSA. 2:16, 1980; F. J. Hills, S. S. Johnson, S. Geng, A.Abshahi, and G. R. Peterson, Calif. Agric. Nov.-Dec., p.

14, 1981; F. J. Hills, S. S. Johnson, S. Geng, A. Abshahi,and G. R. Peterson, Calif. Agric. March-April, p. i7, 1983)ranged from $37.89 to $59.99/103 kg ($34.37 to $54.41/ton)and averaged $51.16/103 kg ($46.40/ton), on a dry feedstockbasis. Growiing costs and crop yields, therefore, have amajor influence on per liter feedstock costs, as do ethanolyields per unit of feedstock mrass.Both growing costs and special drying and storage costs

need to be examined in more detail in future research. Ifdrying costs were to add 20% to feedstock costs, for exam-

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PRODUCTION OF FUEI ETHANOL FROM SWEET SORGHUM

ple, net ethanol costs would increase by $0.06/liter($0.22/gal). This would decrease the attractiveness of sweetsorghum relative to that of corn as an ethanol feedstock. Atthis stage of research, however, it is premature to designatea specific cost for drying of the sweet sorghum.The estimated PF credit for sweet sorghum (Table 3) is

higher than those for corn and fodder beets (3, 5, 6, 22).Thecredit of nearly $0.12/liter of ethanol ($0.45/gal) brings thenet feedstock cost to $0.17/liter, compared with our previousestimates of $0.16/liter for corn and $0.17/liter for fodderbeets. Valuation of the sweet sorghum by-product was basedon the protein content of the raw feedstock (1.98% protein infresh weight sorghum). Our previous work valued distillerswet grain (DWG) on the basis of its use as a proteinsupplement in livestock rations (Hoffman and Dobbs, SDSUAg. Exp. Sta. Bull. 686, 1982). By using the previouslydetermined value of protein in DWG, and comparing theprotein contained in sweet sorghum and corn feedstocks, the$0.12/liter credit was arrived at for sweet sorghum PF. Thissame procedure was also used in our paper on fodder beets(6).

This procedure entails some assumptions that, thoughnecessary at this stage of analysis, are oversimplifications.The protein in sweet sorghum PF will be in a much lessconcentrated form than in the DWG 1qy-product from corn. Itmay thus have less nutritional value per pound of proteinthan does DWG. Because of its greater bulk per pound ofprotein, the sweet sorghum by-product may also have moretransportation, storage, and handling problems than doesDWG. These factors could cause the sweet sorghum by-

TABLE 1. Raw matenals, rates, and products in theoretical farm-scale fuel ethanol plant with sweet sorghuma

ConsumptionComponent Amt/yrb or production

rate (per h)

Raw materialsSweet sorghumc 3.34 x 106 kgd 441 kgd.eWateif 5.0 x 105 litersH2SO4g 50,000 litersYeasth 1,400 kg

Products95% ethanol 658,854 liters' 83 litersPFf 7.7 x 106 kga Plant design and operation are based on preliminary research findings.b A 45-week work year is assumed.c An anhydrous ethanol yield of 179 liters/103 kg of 15% moisture sweet

sorghum i$ assumed here.d The amount per year is the total weight of sweet sorghum (fresh and dried;

see Table 2) required. The value was calculated on a dry weight basis (15%moisture). The consumption rate value is for 11% moisture sweet sorghum.The consumption rate of 72% moisture pulp is 1,339 kg/h.

A 120,000-liter, stainless steel, semicontinuous solid-phase fermentor(Fig. 5) is used for this process. It is assumed that the fermentor is filled to75% capacity and is operated as is shown in Fig. 5. Conveying, weighing, andhammermilling takes 0.1 h; the optional pasteurization step takes 6 h;fermentation takes 48 to 54 h; and pressing-distillation takes 1 h.

f Water is used in preparing the yeast inoculum, washing fermented pulp,rehydrating dry sweet sorghum pulp, and condensing ethanol vapor.

8 3 to 6 ml of concentrated H2SO4 per kg of rehydrated (72% moisture) pulpis used here.

Yeast cells are obtained commercially in a dried, powder form (approxi-mately 10' cells per g).'Amount of denatured ethanol (5 liters of unleaded gasoline per 100 liters of

95% [vol/vol] ethanol) produced per year (5%,106 liters of anhydrous etha-nol).

PF is assumed to be 70 to 75% moisture.

TABLE 2. Energy balance of ethanol production from sweetsorghumEnergy (kJ/liter) produced or consumed'

Parameter Acidified and Acidified and

pasteurized pulp nonpasteurizedpulp

Energy input'Conveyingc 16 16Grinding' 1,087 1,087Pasteurizatione 4,043 0Fermentationf 20 20Pressingg 97 97Distillationh 7,565 7,565Drying-storage' 7,429 7,429

Energy output 21,192 21,192

a Values are expressed as kilojoules per liter of denatured 95% (vol/vol)ethanol. Energy balances for pasteurized and unpasteurized pulp are 1.05 and1.31, respectively.bEnergy input values are based whenever possible (e.g., pasteurization) on

actual plant findings obtained from replicate runs with sweet sorghum. Thevalues for distillation are extrapolated from earlier work on corn (21). Theenergy required for growing the sweet sorghum is not included.

One 0.37-kW motor is required for conveying.d One 25-kW mptor attached to hammermill is required here.e Steam heat is used to pasteurize pulp at 70'C for 6 h.f One 0.5-kW motor is required for rotating auger.g One 0.37-kW motor is required for conveying, and two 0.75-kW motors

are required for pressing.hThree 0.7-kW motors are required for operating pumps; the remainder of

the energy for distillation is in the form of steam.iIn reducing the moisture content of the sweet sorghum billets from 70 to

15% the dryer (2.13 by 9.14 m; Rader Co., Inc., Portland, Oreg.) uses 2.899 kJfor each kilogram of water removed. Waste heat from the plant and heatgenerated by the methane digester qr solar collector (or both) supply 75% ofthe energy required for drying, or 22, 286 kJtliter of 95% ethanol. It is ass'umedthat fresh billets ae4e used during July, August, and September and dry billetsare processed during other months.

i This is the energy-content of 190 proof (95% [vol/voll) ethanol (21). Theoutput energy of the PF is not included.

product to have an actual economic value that is substan-tially less than $0.12/liter of alcohol.There are very few estimates of sweet sorghum by-prod-

uct value in the fuel alcohol literature. One study hassuggested a much lower credit for the sweet sorghum proteinfeed by-product: $0.04/liter of alcohol (M. Meo and S.Sachs, Calif. Agric., 36:9, 1982). This would increase the netcost of the fuel ethanol to about $0.55/liter. The divergencein estimates for the PF further underscores the need foradditional research on sweet sorghum by-product quantities,feeding characteristics, and values.

Despite thp limitations inherent in this preliminary costanalysis, the data provide some basis for feeling that sweetsorghum may be competitive with such other fuel alcoholfeedstocks as corn and fodder beets under at least someclimatic and growing conditions. This is consistent with thefinding of Meo and Sachs in a study based on Californiagrowing conditions (Calif. Agric., 36:9, 1982). They esti-mated fuel alcohol production costs to be lower for the sweetsorghum feedstock than for corn, Jerusalem artichokes,grain sorghum, and fodder beet feedstocks.Our research with corn has shown that certain modifica-

tions in the baseline method of plant operation might reduceethanol costs by as much as $0.11 to $0.13/liter ($0.40 to$0.50/gal) below the costs cited earlier of $0.47/liter($1.78/gal), (Hofftnan and Dobbs, SDSU Ag. Exp. Sta. Bull.686, 1982). Further research with a farm-scale or a largercommunity-scale plant will also be needed to determine

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APPL. ENVIRON. MICROBIOL.

TABLE 3. 1983 costs of ethanol production from sweet sorghumCost ($/liter)'

Parameter Acidified and Acidified andpasteurized nonpasteurized

pulp pulp

Capital costsb 0.047 (0.177) 0.044 (0.169)

Operating costsFeedstockc 0.289 (1.100) 0.289 (1.100)Other 0.215 (0.818) 0.211 (0.803)

Other fixed costsd 0.040 (0.153) 0.040 (0.153)

Credit for PFC 0.118 (0.448) 0.118 (0.448)

Net cost of 0.473 (1.800) 0.466 (1.777)denatured ethanola Costs, which have been derived in most cases from the Dobbs and

coworkers cost breakdown of the plant (3) are given in U.S. dollars per liter ofdenatured 95% (vol/vol) ethanol. Figures in parentheses are costs in U.S.dollars per gallon. Costs for drying and storing the sweet sorghum are notincluded. Data are from the South Dakota State University ethanol plantoperation.

b All capital items are amortized at a rate of 15% over their useful lifetime.c This assumes a sweet sorghum cost of $51.16/103 kg ($46.40/ton) of 15%

moisture sweet sorghum. The feedstock cost is based upon an alcohol yield of179 liters/103 kg (42.9 gal/ton). Wet sweet sorghum (72% moisture) wasconverted to dry sweet sorghum (15% moisture) by the following formula: wetweight x 0.3294 = dry weight.

d This includes insurance, maintenance, and property taxes.PF contains 9o protein (dry weight).

whether the cost of ethanol from sweet sorghum can bereduced a comparable amount from the $0.47/liter figurecalculated in this paper.

ACKNOWLEDGMENTSThis research was made possible through funding from the South

Dakota State University Agricultural Experiment Station andthrough the U.S. Departments of Agriculture and Energy, and EastRiver Rural Electric Cooperative grants.We thank Nancy Thiex, Larry Novotny, Dwayne Beck, Quentin

Kingsley, and Eugene Arnold for technical assistance and advice.We also thank Jodi Finch and Vickie Molengraaf for assistance intyping.

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21. Stampe, S., R. Alcock, C. Westby, and T. Chisholm. 1983.Energy consumption of a farm-scale ethanol distillation system.Energy Agric. 2:355-368.

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