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Encyclopedia of Physical Science and Technology - Biotechnology PDF

298 Pages·2001·10.597 MB·English
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P1:FPP 2ndRevisedPages Qu:00,00,00,00 EncyclopediaofPhysicalScienceandTechnology EN002C-64 May19,2001 20:39 Table of Contents (Subject Area: Biotechnology) Pages in the Article Authors Encyclopedia Biomass Utilization, David Pimentel Pages 159-171 Limits of Biomass, Bruce E. Dale Pages 141-157 Bioengineering of Biomaterials, Carole C. Perry Pages 173-191 Synthetic Synthesis, Fabrication, and Applications Biomineralization and Paul D. Calvert Pages 193-205 Biomimetic Materials Bioreactors Yusuf Chisti and Pages 247-271 Murray Moo-Young Fiber-Optic Chemical David R. Walt, Israel Pages 803-829 Sensors Biran and Tarun K. Mandal Hybridomas, Genetic Michael Butler Pages 427-443 Engineering of Image-Guided Ferenc A. Jolesz Pages 583-594 Surgery Mammalian Cell Bryan Griffiths and Pages 31-47 Culture Florian Wurm Metabolic Engineering Jens Nielsen Pages 391-406 Microanalytical Jerome S. Schultz Pages 679-694 Assays Optical Fiber Abraham Katzir Pages 315-333 Techniques for Medical Applications Pharmaceuticals, Giancarlo Santus and Pages 791-803 Controlled Release of Richard W. Baker Pharmacokinetics Michael F. Flessner Pages 805-820 Separation and Laure G. Berruex and Pages 651-673 Purification of Ruth Freitag Biochemicals Tissue Engineering François Berthiaume Pages 817-842 and Martin L. Yarmush Toxicology in Forensic Olaf H. Drummer Pages 905-911 Science P1:FJURevisedPages Qu:00,00,00,00 EncyclopediaofPhysicalScienceandTechnology EN002C-60 May17,2001 20:23 Biomass Utilization, Limits of David Pimentel CornellUniversity I. BiomassResources II. ConversionofBiomassResources III. Biogas IV. BiomassandtheEnvironment V. SocialandEconomicImpacts VI. Conclusion GLOSSARY denaturant for ethyl alcohol and in the synthesis of otherchemicals. Biodiversity Allspeciesofplants,animals,andmicrobes Pollution The introduction of foreign, usually man- inoneecosystemorworld. made,productsorwasteintotheenvironment. Biogas A mixture of methane and carbon dioxide pro- Pyrolysis Chemicalchangebroughtaboutbytheaction ducedbythebacterialdecompositionoforganicwastes ofheat. andusedasafuel. Subsidy Agrantorgiftofmoney. Biomass Amountoflivingmatter,includingplants,ani- mals,andmicrobes. Energy Energy is the capacity to do work and includes THEINTERDEPENDENCYofplants,animals,andmi- heat,light,chemical,acoustical,mechanical,andelec- crobes in natural ecosystems has survived well for bil- trical. lionsofyearseventhoughtheyonlycaptured0.1%ofthe Erosion The slow breakdown of rock or the movement sun’senergy.Allthesolarenergycapturedbyvegetation andtransportofsoilfromonelocationtoanother.Soil andconvertedintoplantbiomassprovidesbasicresources erosionincropandlivestockproductionisconsidered foralllife,includinghumans.Approximately50%ofthe seriousworldwide. world’sbiomassisusedbyhumansforfoodpluslumber Ethanol Also called ethyl alcohol. A colorless volatile andpulpandmedicines,aswellassupportforallotheran- flammableliquidwiththechemicalformulaC H OH imalsandmicrobesinthenaturalecosystem.Inaddition 2 5 thatistheintoxicatingagentinliquorsandisalsoused somebiomassisconvertedintofuel. asasolvent. Seriousshortagesofbiomassforhumanuseandmain- Methanol Also called methyl alcohol. A light volatile taining the biodiversity in natural ecosystems now exist flammable liquid with the chemical formula CH OH throughout the world. Consider that more than 3 billion 3 that is used especially as a solvent, antifreeze, or humansarenowmalnourished,shortoffood,andvarious 159 P1:FJURevisedPages EncyclopediaofPhysicalScienceandTechnology EN002C-60 May17,2001 20:23 160 BiomassUtilization,Limitsof essential nutrients. This is the largest number and pro- tal biomass production about 36.5 billion metric tons or portionofmalnourishedhumanseverrecordedinhistory. 145×1015 kcal/yr. In contrast, the terrestrial ecosystem Meanwhile,basedoncurrentratesofincrease,theworld producesabout3t/ha/yr,makingthetotalbiomassabout populationisprojectedtodoubletomorethan12billion 40.5billiontonsor162×1015kcal/yr.Thetotalbiomass in approximately 50 years. With a population growth of producedisapproximately77billiontonsorabout12.8t thismagnitude,thenumbersofmalnourishedcouldreach perpersonperyear. 5billionwithinafewdecades.Theneedforbiomasswill The40.5billiontonsofbiomassproducedintheterres- continuetoescalate. trialecosystemprovidesanestimated6.8t/yrperperson. Associated with increasing human numbers are di- Giventhathumansharvestabout50%oftheworld’sterres- verse environmental problems, including deforestation, trialbiomass,eachpersonisutilizing3.4t/yr.This3.4t/yr urbanization, industrialization, and chemical pollution. includesallofagriculture,includinglivestockproduction Allthesechangesnegativelyimpactonbiomassproduc- and forestry. The remaining 3.4 t/yr per person supplies tionthatisvitaltohumanlifeandbiodiversity.However, theother10millionspeciesofnaturalbiotatheirenergy at present and in the foreseeable future the needs of the andnutrientneeds. rapidly growing human population will stress biomass Currently, approximately 50% of the world’s biomass supplies. In our need to supply food and forest products (approximately 600 quads worldwide) is being used by for humans from biomass, intense competition between humans for food, construction, and fuel. This major uti- humanneedsforfoodandtheconversionofbiomassinto lization of biomass, habitat destruction associated with anenergyresourceisexpectedtointensifyinthecoming the rapid increase in the world population, and environ- decades. mental pollution from about 100,000 chemicals used by Furthermore, human intrusion throughout the natural humansiscausingtheseriouslossofbiodiversityworld- environmentiscausingaseriouslossofbiodiversitywith wide.Witheachpassingdayanestimated150speciesare asmanyas150speciesbeinglostperday.Thepresentrate being eliminated because of increasing human numbers ofextinctionofsomegroupsoforganismsis1000–10,000 andassociatedhumanactivities,includingdeforestation, times faster than that in natural systems. Ecosystem and soilandwaterpollution,pesticideuse,urbanization,and speciesdiversityarethevitalreservoirofgeneticmaterial industrialization. for the successful development of agriculture, forestry, pharmaceutical products, and biosphere services in the B. UnitedStatesBiomass future. The limits of biomass energy utilization and how this IntheNorthAmericantemperateregion,thesolarenergy relates to food production and natural biodiversity and reachingahectareoflandperyearis14billionkilocalo- environmentalqualityarediscussedinthisarticle. ries.However,plantsdonotgrowduringthewinterthere. Mostplantgrowthoccursduring4monthsinthesummer whenabout7billionkilocaloriesreachahectare.Inaddi- I. BIOMASS RESOURCES tiontolowtemperatures,plantgrowthislimitedbyshort- agesofwater,nitrogen,phosphorus,potassium,andother Theamountofbiomassavailableislimitedbecauseplants nutrients,plusthefeedingpressureofherbivoresanddis- on average capture only about 0.1% of the solar energy easeorganisms.Atmost,duringawarmmoistdayinJuly reaching the earth. Temperature, water availability, soil aplant,likecorn,underveryfavorableconditions,might nutrients, and feeding pressure of herbivores all limit captureonly5%ofthesunlightenergyreachingtheplants. biomass production in any given region. Under optimal Undernaturalandagriculturalconditionsforthetotalyear, growing conditions, natural and agricultural vegetation vegetationproducesapproximately12millionkilocalories andproduceabout12millionkilocaloriesperhectareper perhectareperyearorabout3t/hadrybiomass. year(about3t/hadrybiomass). TotalannualbiomassproducedintheUnitedStatesis an estimated 2.6 billion tons (Table I). This is slightly morethan6%ofalltheterrestrialbiomassproducedinthe A. WorldBiomass world.BasedontheUnitedStates.landareaof917mil- Theproductiveecosystemsintheworldtotalanestimated lion hectares, this is the equivalent of 2.9 t/ha/yr and is 50billionhectare,excludingtheicecaps.Marineecosys- similartotheworldaverageof3t/ha/yrforalltheterres- temsoccupyapproximately36.5billionhectarewhilethe trial ecosystems of the world. The total energy captured terrestrialecosystemsoccupyapproximately13.5billion by all the United States plant biomass each year is ap- hectare.Grossprimaryproductivityforthemarineecosys- proximately 11.8 × 1015 kcal (Table I). With the United tem is estimated to be about 1 t/ha/yr, making the to- States currently consuming 87 quads (21.8×1015 kcal) P1:FJURevisedPages EncyclopediaofPhysicalScienceandTechnology EN002C-60 May17,2001 20:23 BiomassUtilization,Limitsof 161 TABLEI AnnualBiomassProductionintheUnitedStates Each year, worldwide, an estimated 5300 million dry tons of biomass are burned directly as a fuel, providing Landarea Biomassproduction (106/ha) (106/t) about88quadsofenergy.Ruralpoorindevelopingcoun- tries obtain up to 90% of their energy needs by burning Croplandandcrops 192 1,083 biomass.Indevelopingcountries,about2billiontonsof Pastureandforage 300 900 fuelwood, 1.3 billion tons of crop residues, plus nearly Forests 290 580 1billiontonsofdungareburnedeachyear. Other 135 68 Althoughsomedeforestationresultsfromtheuseoffu- Totalarea 917 — elwood,themostsignificantenvironmentalimpactsresult Totalbiomass — 2,631 fromburningcropresiduesanddung.Whencropresidues Totalenergy(1015/kcal) 11.8 and dung are removed from the land and used as a fuel Biomassproduction(t/ha) 2.9 thisleavesthecroplandwithoutvegetativeprotectionand exposedtowindandwatererosion.Erosiondestroysthe [FromPimentel,D.,andKounang,N.(1998),Ecosystems1,416–426.] productivityofcropland,byrobbingthesoilofnutrients, essentialwater,soilorganicmatter,andadequaterooting offossilenergyeachyear,thismeansthatitisconsuming depth. 85%morefossilenergythanthetotalenergycapturedby Cookingrequiresrelativelylargeamountsoffuelandis allitsplantbiomasseachyear. essentialforpreventingdisease,improvingnutrition,and increasing the palatability of many foods. The transfer C. UnitedStatesAgriculturalandForest of heat from the woodfire in a stove to the food product ProductsandBiofuels is about 33% efficient, while over an open fire, the heat transfer to the food is only about 10% efficient. Under Including crops and forages from pastures, the United usualcookingconditions,from2to3kcalarerequiredto States harvests approximately 1307 million tons of cook1kcaloffood. biomass per year in agricultural products and approxi- mately 100 million tons of biomass per year as forest products (Table II). Together the energy value of harvested TABLEII TotalAnnualAmountofSolarEnergyHarvestedin agricultural and forest products total 63521012 kcal/yr theFormofAgriculturalandForestBiomassintheU.S. (Table II). These data suggest that the United States is Tons(106) Energy(1012kcal) harvestingintheformofagriculturalandforestproducts, 54%ofthetotalenergycapturedeachyearbytheUnited Corn 194 873 States biomass annually (Tables I and II). This total does Wheat 71 320 notincludethebiomassharvestednowandusedasbiofuel. Rice 6 27 Soybeans 51 230 Sorghum 22 99 II. CONVERSION OF BIOMASS Potatoes 16 72 RESOURCES Vegetables 6 27 Fruits 5 23 Inadditiontousingbiomassdirectlyasfood,fiber,lumber, Nuts 0.8 4 and pulp, biomass is utilized as a fuel. The total biofuel Oilseeds 9 41 utilizedintheUnitedStatesisslightlymorethan3quads Sugarcane 2.5 20 (800×1012 kcal)peryear.Ifthebiofuelenergyisadded Sugarbeets 2 27 tothatharvestedasagriculturalandforestproducts,then Pulses 1 5 thetotalbiomassenergyharvestedfromtheUnitedStates Oats 7 32 terrestrialecosystemis7332×1012kcal/yr.Thisisequiv- Rye 1 5 alentto62%ofthetotalbiomassenergyproducedinthe Barley 13 59 UnitedStateseachyear.Harvestingthis62%ishavinga Total 407.3 1,853 negativeimpactonbiodiversityinthenation. Pastureforage 900 4,050 Forestproducts 100 450 A. DirectHeating Totals 1,407 6,352 Totalpercapita(tons) 5.2 Heat production is the most common conversion system Totalpercapita(106/kcal) 23.3 for using biomass resources. Heat from wood and other biomass resources is utilized for cooking food, heating [FromPimentel,D.,andKounang,N.(1998),Ecosystems1,416– homes,andproducingsteamforindustry. 426.] P1:FJURevisedPages EncyclopediaofPhysicalScienceandTechnology EN002C-60 May17,2001 20:23 162 BiomassUtilization,Limitsof Inadevelopingcountryanaverage,600–700kg/yrof combustionreleasesmorethan200differentchemicalpol- drybiomassperpersonisusedforcooking.Forexample, lutantsintotheatmosphere.Thepollutantsinclude,upto the use of fuelwood for cooking and heating in Nepal is 14carcinogens,4cocarcinogens,and6toxinsthatdam- about846kg/yrofbiomassperperson.Otherinvestigators agecilia,plusadditionalmucus-coagulatingagents.Wood reportthatfrom912to1200kg/yrofbiomassperperson smokecontainspollutantsknowntocausebronchitis,em- isusedforbothcookingandheating.Insomedeveloping physema,cancer,andotherseriousillnesses. countries,fuelwoodforcookingandheatingmaycostal- Globally, but especially in developing nations where mostasmuchasthefood,makingitnecessarytousecrop peoplecookwithfuelwoodoveropenfires,approximately residuesanddung. 4 billion humans suffer continuous exposure to smoke. A significant amount of wood is converted into char- Thissmokewhichcontainslargequantitiesofparticulate coal for cooking and heating. Similar to wood fires for matterandmorethan200chemicals,includingseveralcar- cooking,opencharcoalfiresareonlyabout10%efficient cinogens,resultsinpollutionlevelsthatareconsiderably intransferringheatenergytofood.However,charcoalhas abovethoseacceptablebytheWorldHealthOrganization some advantages over wood. First, it is lightweight and (WHO).Worldwidefuelwoodsmokeisestimatedtocause easytotransport.Onekilogramofcharcoalcontainsabout the death of 4 million children each year worldwide. In 7100kcalofpotentialenergyincontrasttoakilogramof India,wherepeoplecookwithfuelwoodanddung,partic- woodthathasabout4000kcal.Charcoalburnsmoreuni- ulateconcentrationsinhousesarereportedtorangefrom formlyandwithlesssmokethanwood. 8300 to 15,000 µg/m3, greatly exceeding the 75 µg/m3 However, charcoal production is an energy-intensive maximum standard for indoor particulate matter in the process. Although charcoal has a high energy content, UnitedStates. from 20,300 to 28,400 kcal of hardwood must be pro- Because of the release of pollutants, some communi- cessed to obtain the 7100 kcal of charcoal. Considering tiesindevelopedareas,suchasAspen,CO,havebanned this low conversion efficiency ranging from 25 to 35%, woodburningforheatinghomes.Whenbiomassisburned charcoalheatingforcookinghasanoverallenergytrans- continuouslyinaconfinedspaceforheating,itspollutants ferefficiencytofoodofonly2.5–3.5%.Further,theuseof accumulateandcanbecomeaserioushealththreat. charcoal uses more forest biomass than directly burning thewood. C. EthanolProduction Usingfuelwoodfortheproductionofsteaminaboiler under relatively optimal conditions is 55–60% efficient, Numerousstudieshaveconcludedthatethanolproduction thatis,burning4000kcalofair-driedwoodprovidesfrom doesnotenhanceenergysecurity,isnotarenewableen- 2200to2400kcalofsteamintheboiler.Moreoftenthe ergysource,isnotaneconomicalfuel,anddoesnotinsure efficiencyislessthan55–60%.Steamproductionisused cleanair.Further,itsproductionuseslandsuitableforcrop to produce electricity and producing a salable product, productionandcausesenvironmentaldegradation. suchassteam,forindustrialuse. Theconversionofcornandotherfood/feedcropsinto Collecting biomass for fuel requires a substantial ethanolbyfermentationisawell-knownandestablished amount of time and human effort. For example, in technology.Theethanolyieldfromalargeplantisabout Indonesia,India,Ghana,Mozambique,andPerufamilies 9.5l(2.5gal)fromabushelofcornof24.5kg(2.6kg/l spendfrom1.5to5hrseachdaycollectingbiomasstouse of ethanol). Thus, a hectare of corn yielding 7965 kg/ha asafuel. couldbeconvertedintoabout3063lofethanol. Estimatesarethatmorethanhalfofthepeoplewhode- TheproductionofcornintheUnitedStatesrequiresa pendonfuelwoodhaveinadequatesupplies.Insomecoun- significant energy and dollar investment (Table III). For tries,suchasBrazil,whereforestareasareatpresentfairly example, to produce 7965 kg/ha of corn using conven- abundant,theruralpoorburnmostlywoodandcharcoal. tionalproductiontechnologyrequirestheexpenditureof However,inmanydevelopingcountriescropresiduesac- about10.4millionkcal(about10,000lofoilequivalents) count for most of the biomass fuel, e.g., 55% in China, (Table III), costing about $857.17 for the 7965 kg or ap- 77%inEgypt,and90%inBangladesh.Estimatesarethat proximately10.8c//kgofcornproduced.Thus,foraliter thepoorinthesecountriesspend15–25%oftheirincome ofethanol,thecornfeedstockalonecosts28c/. forbiomassfuel. Thefossilenergyinputtoproducethe7965kg/hacorn feedstock is 10.4 million kilocalories or 3408 kcal/l of ethanol (Table III). Although only 16% of United States B. HealthEffects corn production is currently irrigated, it is included in Environmentally,burningbiomassismorepollutingthan theanalysis,becauseirrigatedcornproductionisenergy using natural gas, but less polluting than coal. Biomass costly. For the 150 mm of irrigation water applied and P1:FJURevisedPages EncyclopediaofPhysicalScienceandTechnology EN002C-60 May17,2001 20:23 BiomassUtilization,Limitsof 163 TABLEIII EnergyInputsandCostsofCornProductionper quired to produce 1000 l of ethanol than the energy that HectareintheUnitedStates actually is in the ethanol (Table IV). Inputs Quantity kcal××1000 Costs In the distillation process, large amounts of fossil en- ergy are required to remove the 8% ethanol out of the Labor 11.4hr 561 $100.00 92%water.Forexample,toobtain1000lofpureethanol Machinery 55kg 1,018 103.21 withan8%ethanolconcentrationoutof92%water,then Diesel 42.2L 481 8.87 thisethanolmustcomefromthe12,500lofethanol/water Gasoline 32.4L 328 9.40 mixture.Atotalof124lofwatermustbeeliminatedper Nitrogen 144.6kg 2,668 89.65 literofethanolproduced.Althoughethanolboilsatabout Phosphorus 62.8kg 260 34.54 78◦C,incontrasttowaterat100◦C,theethanolisnotex- Potassium 54.9kg 179 17.02 tractedfromthewaterinonedistillationprocess.Instead, Lime 699kg 220 139.80 about 3 distillations are required to obtain the 95% pure Seeds 21kg 520 74.81 ethanolthatcanbemixedwithgasoline.Tobemixedwith Herbicides 3.2kg 320 64.00 gasoline,the95%ethanolmustbefurtherprocessedwith Insecticides 0.92kg 92 18.40 more energy inputs to achieve 99.8% pure ethanol. The Irrigation 150mm 3,072 150.00 threedistillationsaccountforthelargequantitiesoffos- Electricity 13.2kg 34 2.38 silenergythatarerequiredinthefermentation/distillation Transportation 151kg 125 45.30 process.Note,inthisanalysisalltheaddedenergyinputs Total 10,439 $857.17 forfermentation/distillationprocessareincluded,notjust Cornyield 27,758 thefuelforthedistillationprocessitself. =7,965kg kcaloutput/kcalinput=1:2.66 This contrasts with Shapouri et al. who, in 1995, give only one figure for the fermentation/distillation process FromPimentel,D.,Doughty,R.,Carothers,C.,Lamberson,S.,Bora, N.,andLee,K.J.Agr.Environ.Ethics(inpress). and do not state what the 3.4 million kilocalories repre- sents in their analysis for producing 1000 l of ethanol. Careful and detailed analyses and full accountings are pumpedfromonly30.5m(100feet),theaverageenergy neededtoascertainthepracticalityofethanolproduction input is 3.1 million kilocalories/hectare (Table III). asaviableenergyalternative. When investigators ignore some of the energy inputs About 61% of the cost of producing ethanol (46c/ per in biomass production and processing they reach an in- liter)insuchalarge-productionplantisforthecornsub- completeanddeficientanalysisforethanolproduction.In strate itself (28c/ /l) (Table IV). The next largest input is for a recent USDA report, no energy inputs were listed for coaltofuelthefermentation/distillationprocess,butthis machinery, irrigation, or for transportation. All of these was only 4c/ (Table IV). These ethanol production costs are major energy input costs in United States corn pro- includeasmallchargeforpollutioncontrol(6c/ perliter), duction (Table III). Another way of reducing the energy whichisprobablyalowestimate.Insmallerplantswith inputsforethanolproductionistoarbitrarilyselectlower anannualproductionof150,000l/yr,thecostperliterin- production costs for the inputs. For instance, Shapouri creasestoasmuchas66c/ perliter.Overall,theperliter etal.listthecostofakilogramofnitrogenproductionat 12,000 kcal/kg, considerably lower than Food and Agri- culturalOrganizationoftheUN(FAO),whichlistthecost TABLEIV Inputsper1000lofEthanolProducedfromCorn ofnitrogenproductionat18,590kcal/kg.Usingthelower Inputs Kilograms Kilocalories(1000) Dollars figure reduces the energy inputs in corn production by about 50%. Other workers have used a similar approach Corn 2,600 3,408 $280 tothatofShapourietal. Transportofcorn 2,600 312 32 Theaveragecostsintermsofenergyanddollarsfora Water 160,000 90 20 large(240to280millionlitersperyear),modernethanol Stainlesssteel 6 89 10 plant are listed in Table IV. Note the largest energy in- Steel 12 139 10 putsareforcornproductionandforthefuelenergyused Cement 32 60 10 in the fermentation/distillation process. The total energy Coal 660 4,617 40 inputtoproduce1000lofethanolis8.7millionkilocalo- Pollutioncontrolcosts — — 60 ries (Table IV). However, 1000 l of ethanol has an energy Total 8,715 $462 valueofonly5.1millionkilocalories.Thus,thereisanet From Pimentel, D., Warneke, A. F., Teel, W. S., Schwab, K. A., energylossof3.6millionkilocaloriesper1000lofethanol Simcox,N.J.,Ebert,D.M.,Baenisch,K.D.,andAaron,M.R.,(1988). produced.Putanotherway,about70%moreenergyisre- Adv.Food.Res.32,185–238. P1:FJURevisedPages EncyclopediaofPhysicalScienceandTechnology EN002C-60 May17,2001 20:23 164 BiomassUtilization,Limitsof priceforethanoldoesnotcomparefavorablywiththatfor Furthermore, some of the economic and energy con- theproductionofgasolinefuelswhichpresentlyisabout tributionsoftheby-productsarenegatedbytheenviron- 25c/ perliter. mentalpollutioncostsassociatedwithethanolproduction. Basedoncurrentethanolproductiontechnologyandre- These are estimated to be about 6c/ per liter (Table IV). In centoilprices,ethanolstillcostssubstantiallymoretopro- UnitedStatescornproduction,soilerodesabout12times duceindollarsthanitisworthonthemarket.Clearly,with- faster than it can be reformed. In irrigated corn acreage, out the approximately $1 billion subsidy, United States ground water is being mined 25% faster than its natural ethanolproductionwouldbereducedorcease,confirming rechargerate.Thissuggeststhattheenvironmentalsystem thefactthatbasicallyethanolproductionisuneconomical. inwhichcornisbeingproducedisbeingrapidlydegraded. Federalsubsidiesaverage16c/ perliterandstatesubsidies Further,itsubstantiatesthefindingthattheUnitedStates average5c/ perliter.Becauseoftherelativelylowenergy corn production system is not sustainable for the future, contentofethanol,1.5lofethanolistheenergyequivalent unless major changes are made in the cultivation of this of1lofgasoline.Thismeansthatthecostofsubsidized major food/feed crop. Corn should not be considered a ethanol is 68c/ per liter. The current cost of producing renewableresourceforethanolenergyproduction. gasolineisabout25c/ perliter. Whenconsideringtheadvisabilityofproducingethanol Atpresent,federalandstatesubsidiesforethanolpro- forautomobiles,theamountofcroplandrequiredtogrow ductiontotalabout$1billionperyearandaremainlypaid corn to fuel each automobile should be understood. To tolargecorporations(calculatedfromtheabovedata).The clarifythis,theamountofcroplandneededtofueloneau- costs to the consumer are greater than the $1 billion per tomobilewithethanolwascalculated.AnaverageUnited yearusedtosubsidizeethanolproductionbecauseofin- States automobile travels about 16,000 km/yr and uses creasedcornprices.Theresultinghighercornpricestrans- about1900l/yrofgasoline.Although8000kg/haofcorn late into higher meat, milk, and egg prices because cur- willyieldabout3100lofethanol,ithasanenergyequiv- rentlyabout70%ofthecorngrainisfedtoUnitedStates alent of only 1952 l because ethanol has a much lower livestock.Doublingethanolproductioncanbeexpectedto kilocaloriescontentthangasoline. inflatecornpricesperhapsasmuchas1%.Therefore,in However,evenassumingzeroornoenergychargefor additiontopayingtaxdollarsforethanolsubsidies,con- thefermentationanddistillationprocessandchargingonly sumerswouldbepayingsignificantlyhigherfoodprices for the energy required to produce corn (Table III), the net inthemarketplace.ItshouldbenotedthattheUSDAis fuelenergyyieldfrom1haofcornis433l.Thus,topro- proposing to increase the subsidies to the large corpora- vide1900lpercar,about4.4haofcornmustbegrownto tionsbyabout$400millionperyear. fuelonecarwithethanolforoneyear.Incomparison,only Currently about 3.8 billion liters of ethanol are being 0.6haofcroplandiscurrentlyusedtofeedeachAmerican. producedintheUnitedStateseachyear.Thisamountof Therefore,morethanseventimesmorecroplandwouldbe ethanol provides only about 1% of the fuel utilized by required to fuel one automobile than is required to feed UnitedStatesautomobiles.Toproducethe3.8billionliters oneAmerican. ofethanolwemustuseabout1.3millionhectaresofland. Assuming a net production of 433 l of fuel per corn If we produced 10% of United States fuel the land re- hectare and if all automobiles in the United States were quirementwouldbe13millionhectares.Moreovernotall fueled with ethanol, then a total of approximately 900 the3.8billionliterswouldbeavailabletouse,becausea million hectares of cropland land would be required to lotwouldbeneededtosow,fertilize,andharvest13mil- provide the corn feedstock for production. This amount lionhectares.Clearly,cornisnotarenewableresourcefor ofcroplandwouldequalnearlythetotallandareaofthe ethanolenergyproduction. UnitedStates. The energy and dollar costs of producing ethanol can Brazil had been a large producer of ethanol, but has beoffsetinpartbytheby-productsproduced,especially abandonedsubsidizingit.Withoutthesubsidy,economic thedrydistillersgrains(DDG)madefromdry-millingthat ethanolproductionisimpossible. canbefedprimarilytocattle.Wet-millingethanolplants producesuchby-productsascornglutenmeal,glutenfeed, and oil. Sales of the by-products help offset the energy III. BIOGAS andeconomic costs of ethanolproduction.For example, use of by-products can offset the ethanol production costs Biomass material that contains large quantities of water by 8–24% (Table IV). The resulting energy output/input canbeeffectivelyconvertedintousableenergyusingnat- comparison, however, remains negative (Table IV). The urallyoccurringmicrobesinananaerobicdigestionsys- salesoftheby-productsthatrangefrom13to16c/ perliter tem.Thesesystemsusefeedstocks,likedungandcertain donotmakeethanolcompetitivewithgasoline. plants such as water hyacinth, although production and P1:FJURevisedPages EncyclopediaofPhysicalScienceandTechnology EN002C-60 May17,2001 20:23 BiomassUtilization,Limitsof 165 harvesting costs of the latter are generally greater than biogas.Thisefficiencyvariesfrom18to95%.Dairycows fordung.Theprocessingfacilitycanberelativelysimple produce 85 kg daily of manure for each 1000 kg of live and be constructed for about $700. A large facility ca- weight. The total solids in this manure average 10.6 kg, pable of processing the dung from 320 cows might cost andofthese,8.6kgareVS.Theoretically,a100%efficient about$150,000.Thebasicprinciplesforbothsystemsare digestercouldproduce625lofbiogasforeverykilogram similar. ofVSinthesystem.Thedigesterutilizedforthedatapre- Manure from a dairy farm or small cattle operation is sented in Table V was 28.3% efficient. It produces 177 l of loadedorpumpedintoasealed,corrosion-resistantdiges- biogasperkilogramofVSaddedor1520lofbiogasper tion tank where it is held from 14 to 28 days at temper- 1000kgliveweightofcattledaily.Note,ifthetotalheat atures from 30 to 38◦C. In some digestion systems, the value of the manure was used in calculating efficiency, manureinthetankisconstantlystirredtospeedthediges- thenthepercentageefficiencywouldbeonly5%. tionprocessandassureevenheating.Duringthisperiod, Biogas has an energy content of about 5720 kcal/m3, themesophilicbacteriabreakdownvolatilesolids(VS)in comparedto8380kcal/m3forpuremethanegas,because themanureandconvertthemintomethanegas(65%)and carbondioxideispresentinthebiogas.Energycostsand carbon dioxide (35%). Small amounts of hydrogen sul- energyoutputsforprocessing100tofmanure(wet),with fidemayalsobeproduced.Thisgasisdrawnoffthrough a7.1millionkilocaloriesenergyinput,resultsinatotalof pipesandeitherburneddirectly,similartonaturalgas,or 10.2millionkilocaloriesproducedforanetenergyyield scrubbedtocleanawaythehydrogensulfideandusedto of 3.1 million kilocalories (Table V). Much of the energy generate electricity. The energy output/input is listed in inputorcostcomesfromtheproductionofelectricityto Table V. runthepumpsandstirringsystemusedtoreducethere- Theamountofbiogasproducedinthissystemisdeter- tention time in the digester. The volume of the digester mined by the temperature of the system, the VS content isdeterminedbytheamountofmanureproducedbythe of the feedstock, and the efficiency of converting it into animalsduringtheretentiontime.Inthisexample,witha retentiontimeof14days,itwouldbeslightlyover75m3. It is assumed that the electricity is generated from the biogasandthattheelectricalconversionefficiencyofthe TABLEV EnergyInputsUsingAnaerobicDigestionforBio- gasProductionfrom100twet(13tdry)usingCattleManure entireoperationis33%.Theenergyneededtoheatthedi- (Pimentel et al., 1988)a,b gesteriscogeneratedbytheelectricgeneratorviatheuse ofthegenerator’scoolingsystemastheheatsource.The Quantity kcal(1,000) netenergyproducedbythedigestercaneitherbeusedto Inputs generateelectricityforthefarmorbeusedasheatsource Laborhours 20hr — forotheron-farmactivities. Electricity 2,234kWh 5,822 Althoughmaterialcostsareloweredifthereisnogen- Cementfoundation(30-yearlife) 0.9kg 2 erator or stirring mechanism on the digester, the size of Steel(gascollectorandother 35kg 725 thedigestermustbeincreasedbecauseoftheincreasedre- equipmentwith30-yearlife) tentiontimeneededtocompletetheprocess.Also,some Pumpsandmotors 0.5kg 1 ofthebiogaswillhavetobeusedtoheatthedigester,per- Truck/tractorfortransport 10kg 200 hapsanadditional610,000kcalforevery100wettonsof (10-yearlife) manuredigested.Thecriticalheatrequirementsarecalcu- Fuelfortransport(10-kmradius) 34l 340 latedbyincludingtheheatlossestothesurroundings,the Totalinputs 7,090 heat associated with the feed and effluents, and the heat Totalbiogasoutput 10,200 releasedbythebiologicalreaction.Inthetropics,theover- aTheretentiontimeinthedigesteris20days.Theunithasthecapacity all efficiency of the biogas systems is enhanced because toprocess1,825t(wet)peryear.Note:theyieldinbiogasfrom100tis thereisnoneedtoheatthesystemtokeepthetemperature estimatedat10.2millionkilocalories.Thus,thenetyieldis3.1million inthe30–38◦Crange. kilocalories.Theenergyforheatingthedigesteriscogeneratedfromthe Dairycattlearenottheonlysourceofmanureforbio- coolingsystemoftheelectricgenerator. bItisassumedthatanaerobicdigestionofthemanuretakesplaceat gassystems.Theyareusedasamodelsincedairyanimals 35◦Cwithasolidsretentiontimeof20days.Thetemperatureofthe aremorelikelytobelocatedinacentralizedsystem,mak- freshmanureis18◦C,andtheaverageambienttemperatureis13◦C. ing the collecting and adding the manure to a digestion Themanureisassumedtohavethefollowingcharacteristics:production systemlesstimeconsumingandenergyintensivethanfor percowperday,23.6kgtotal;solids,3.36kg;andbiologicaloxygen range-fed steers, or even for draft animals. Efficiencies demand(BOD),0.68kg.Thedigesterisassumedtotransform83%ofthe of conversion vary not only from system to system, but biodegradablematerialintogas.Thebiogasproducedis65%methane, anditsheatofcombustionis5720kcal/m3atstandardconditions. alsothesourcesofmanure.Swineandbeefcattlemanure P1:FJURevisedPages EncyclopediaofPhysicalScienceandTechnology EN002C-60 May17,2001 20:23 166 BiomassUtilization,Limitsof appearstoyieldmoregasperkilogramofVSthandairy TABLEVI EnergyInputsforAnaerobicDigesterintheTrop- cattlemanure.Poultrymanureisalsoused,butsandand icsforBiogasProductionusing8t(1tdry)ofCowManure (Pimentel et al., 1988)a otherformsofheavygritinthisdungcausepumpmain- tenance problems and require more frequent cleaning of Quantity(kg) kcal thedigester. Inputs Manure processed in the digester retains its fertilizer Cementfoundation(30-yearlife) 0.07 140 valueandhastheadvantageoflessodor.Therefore,itcan Steel(30-yearlife) 0.33 7,000 bespreadonfieldsandmaybeeasiertopumpiftheini- Totalinputs 7,140 tialpumpingsystemusedacutterpumptobreakupstray Totalbiogasoutput 820,000 bits of straw or long undigested fibers. Biogas systems Netreturnper1tdrymanure 812,840 havetheadvantageofbeingabletoadjustinsizeaccord- ing to the scale of the operation. The pollution problem aTheretentiontimeis20dayswithoutameansofstoringthebiogas. associatedwithmanureinacentralizeddairyproduction Thegasisusedasdelivered.Thedigestiontakesplaceat35◦C.The systemisthesamewhetherornotitgoesthroughabiogas temperatureofthefreshmanureisassumedtobe21◦C,andtheaverage ambient temperature is 21◦C. The efficiency of the digester is 25%. generator. The biogas produced is 65% methane and its heat of combustion is Indevelopingcountries,suchasIndia,thesituationis 5720kcal/m3. different.There,asubstantialpercentageofthemanureas dried dung is burned directly as fuel. Although burning utilizes a significantly higher percentage of the total en- anestimated2277m3 ofbiogasperyearataconversion ergyinthemanure,itresultsinacompletelossofnitrogen efficiency of 25% (Table VI). The energy value of this andlossofsubstantialamountsoftheothervaluablenutri- gastotals13.0millionkcal.Assuming$8.38per1million ents.Whetherornotbiogasisausefulenergyalternative kcal,theeconomicvalueofthismuchenergyis$109per in India and other similar countries is highly problem- year. Then if no charge is made for labor and dung and aticinspiteofthehigheroverallenergyefficiencyofthe thecapitalcostisassumedtobeonly$14peryear,thenet conversionsystem. returnis$95peryear.Thesecostsarenotequallyappli- Ifitisnotdesirabletoproduceelectricityfromthebio- cable to Kenya where the energy replacement of biogas gas, the energy data listed in Table V will change consider- intermsofwoodfuelsavedisappropriate.Usinganaver- ably.Forinstance,lessenergywillbelostintheconversion age of 4000 kcal/kg of woodfuel, this amount of biogas toelectricityifalltheenergyisuseddirectlyforheating. would replace 3 t of wood and since biogas is generally However,compressingbiogasforuseintractorsinvolves moreefficientthanwoodwhenusedforcooking,thetotal the input of significant amounts of additional energy for amountofwoodreplacedmightbedouble. “scrubbing” the biogas to remove hydrogen sulfide and Althoughthelaborrequirementforthedescribedbio- water. gasgeneratorisonly5–10min/day,thelaborinputforcol- lectingandtransportingbiomassforthegeneratormaybe significant.Ifthesourceforthe400kgofmanurerequired A. BiogasforSmallholders forthedigesterwas,onaverage,3kmfromthedigester, Theeconomicsofbiogasproductioninaruralareaofade- it would take 2 laborers working an 8-hr day to collect velopingnation,likeKenyaorIndia,illustratesthatcosts manure,feeditintothedigester,andreturnthemanureto and benefits are complex and results mixed. The capital croplandwhereitcouldbeutilizedasfertilizer.Onaper costsofconstructingasimplebiogasdigesterwithaca- hourbasis,thelaborerswouldhavetoworkfor3c/ perhour pacitytoprocess8t(wet)ofmanureper20-dayretention forthebiogasdigestertohavecostsequaltotheamount time, or 400 kg/day, are estimated to be between $2000 ofgasproduced.Insomesituations,especiallyindensely and $2500 (Table VI). Such a unit would have usable life populatedpartsofacountry,theamountoftransportre- of30years,sothecapitalcostsareonly$80peryear. quiredwillbetoocostly. If rural workers construct the biogas generator them- Althoughtheprofitabilityofsmall-scalebiogasproduc- selves,materialcostsmightrangefrom$300to$700.At tionmaybelowevenwithoutthechargeoflabor,biogas $400 for materials, without any charge for labor, the in- digesters have significant advantages in rural areas. The vestmentwouldbeonly$14peryearwiththecostsspread biomasscanbeprocessedandfuelenergyobtainedwith- outoverthelifeofthedigester. out losing the valuable nutrients (N, P, and K) present AdigesterthissizeinIndia,wherecowsweighanaver- inthemanure.Nitrogenandphosphorusaremajorlimit- ageofbetween225to330kgeach,wouldrequireaccessto ingnutrientsintropicalagricultureandthesearereturned manurefromabout20cows.Thissystemwouldproduce tothecropland.Theonlylossthattheprocessedmanure

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