Cradle-to-Gate Life Cycle Assessment Of Switchgrass Fuel .
CRADLE-TO-GATE LIFE CYCLE ASSESSMENT OF SWITCHGRASSFUEL PELLETS MANUFACTURED IN THE SOUTHEASTERNUNITED STATES1R. D. Bergman*{Research Forest Products TechnologistEconomics, Statistics, and Life Cycle Analysis UnitForest Products LaboratoryUSDA Forest ServiceMadison, WI 53726-2398E-mail: rbergman@fs.fed.usD. L. ReedEnvironmental ScientistTennessee Department of Environment and ConservationJackson, TN 38305E-mail: Daniel.L.Reed@tn.govA. M. Taylor{Associate Professor and Wood Products Extension SpecialistE-mail: mtaylo29@utk.eduD. P. HarperAssociate ProfessorE-mail: dharper4@utk.eduD. G. HodgesProfessorDepartment of Forestry, Wildlife & FisheriesUniversity of TennesseeKnoxville, TN 37920E-mail: dhodges2@utk.edu(Received September 2014)Abstract. Developing renewable energy sources with low environmental impacts is becoming increasingly important as concerns about consuming fossil fuel sources grow. Cultivating, harvesting, drying,and densifying raw biomass feedstocks into pellets for easy handling and transport is one step forward inthis endeavor. However, the corresponding environmental performances must be quantified. This studypresents cradle-to-gate life cycle inventory and impact assessment data for switchgrass fuel pelletspotentially manufactured in the US Southeast. Because there are no current manufacturers of switchgrasspellets, inventory data were based on field trials of cultivation and harvest of switchgrass combined with aseparate study of wood pelletization. Energy inputs for cultivation and harvest of switchgrass were collectedby survey from farmers in Tennessee and represent the years 2008, 2009, and 2010. Data for pelletizationwere taken from a report on wood pellet manufacturing in the US Southeast. To produce 1.0 Mg of pellets* Corresponding author{ SWST member1This article was written and prepared by US Government employees on official time, and it is therefore in the publicdomain and not subject to copyright.Wood and Fiber Science, 47(2), 2015, pp. 147-159# 2015 by the Society of Wood Science and Technology
148WOOD AND FIBER SCIENCE, APRIL 2015, V. 47(2)that contain 18.0 GJ of potential bioenergy, 4.1 GJ of fossil energy inputs were required. Switchgrasscrops require relatively little energy and inputs for the cultivation and harvest processes. The majority ofthe environmental burdens are associated with drying and pelletizing the raw material.Keywords:Switchgrass, LCA, biomass, pellets, environmental impacts, life cycle inventory, Tennessee.INTRODUCTIONMany factors are motivating the development ofbiofuels and bioproducts: high petroleum prices,a desire for energy independence, the need forrural economic diversification, and concern aboutthe environmental impacts of using fossil carbonsources (Kojima and Johnson 2006; Lee et al 2008;Peters and Thielmann 2008). Regarding the lastpoint, products and fuels made from plants inherently have environmental advantages: they arerenewable and solar-powered, and when sustainably harvested, their use is carbon-neutral togreenhouse gas (GHG) concentrations and globalclimate change. There is growing debate aboutthese potential environmental benefits, however,and more attention is being paid to the amountof fossil carbon resources consumed in the production and processing of bioenergy and thepotential tradeoffs involved (eg between foodand fuel). Although the environmental advantages of biobased resources remain important,they can no longer be assumed. They must bedemonstrated using generally accepted methods.Life cycle assessment (LCA) is an internationally accepted standard method for evaluating theenvironmental impacts of processes and products (ISO 2006a). This LCA study focuses onswitchgrass (Panicum virgatum L.).Switchgrass is indigenous to Central and NorthAmerica. In the southeastern United States (SE),this perennial crop has a growing season fromMay through September and can reach up to2.4 m tall. Switchgrass along with other perennialgrasses and crops are being considered as newsources for bioenergy and bioproducts. In addition, switchgrass is attractive because it requireslittle fertilization for high productivity, possessesa high gross calorific value, is noninvasive, andcan be cultivated on marginal lands, thus limitingcompetition with food production (Wilson et al2012; Brassard et al 2014; Daystar et al 2014;Quinn et al 2014). Also, several studies havereported that switchgrass production can be fullysustainable (McLaughlin and Walsh 1998;McLaughlin et al 2002). Still, under intensivemanagement practices for the biofuel industry,sustainable production as well as maximum productivity of switchgrass will probably requirenitrogen if not phosphorous fertilization (Muiret al 2001; Karp and Shield 2008; Mooney et al2009). Therefore, it is expected that throughselective breeding, highly productive switchgrassspecies with high nitrogen-use efficiency will becreated, thus lowering the need for nitrogenfertilization and increasing both yield and soilcarbon sequestration and perhaps improvingsite fertility (McLaughlin and Walsh 1998;McLaughlin et al 2002; Blanco-Canqil 2010;Wright and Turhollow 2010). Because thesecrops are harvested seasonally and are bulky tohandle and transport, densification processes suchas pelletization may be necessary as a primaryprocessing step (Wilson et al 2012). Pellets aredesigned to be dry, dense, easily handled, andstable in long-term storage.Pellets are burned directly for fuel or can bean initial processing step in a biorefinery or biofuel conversion process. Wood pellets are anestablished fuel product that is growing in importance in the United States and abroad, driven byrising fuel prices and demands for green energysources. Switchgrass pellets have the potentialto join wood pellets in this growing market. Astudy by Sultana and Kumar (2012) evaluatedand ranked biomass feedstock-based pellets witha multicriteria assessment model based on environmental, economical, and technical factors.Wood pellets were named the best among thefive alternatives evaluated, closely followedby switchgrass pellets. Particularly in the Southeast, interest in switchgrass has been establishedand the crop is currently being cultivated and
Bergman et al—SWITCHGRASS FUEL PELLETS CRADLE-TO-GATE LCAharvested for energy and research purposes(Qualls et al 2012; Daystar et al 2014).Some LCA data on switchgrass or similar croppellet production have been published forCanadian and European contexts (Jannasch et al2001; Tilman et al 2006; Smeets et al 2009;Cherubini and Jungmeier 2010; Kalita 2012;Li and Mupondwa 2012). This study focuses onthe US Southeast. The data are intended foranalyses of pelletized biofuels and for relatedproducts (eg when switchgrass or pellets are acomponent of a biorefinery).METHODSGoalThe goal was to develop an inventory of theinputs and outputs associated with the productionof switchgrass pellets in the US Southeast for2010. Results were used in life cycle inventoryassessment (LCIA) and interpretation phases toidentify major sources of environmental impactsand to compare the environmental impacts ofswitchgrass pellets with other energy sources(eg natural gas). The output of this study isintended for use by researchers and practitionersas an input to the life cycle analysis of biomass materials.ScopeThe scope of this study was a cradle-to-gateLCA of switchgrass pellets and includes datafrom cultivation and harvest of switchgrassplantations established in east Tennessee as partof the University of Tennessee Biofuels Initiative (UTBI) (2008). Funded by a 70.5 millionstate investment, this initiative has been chargedwith developing a cellulosic biofuels industry inTennessee. Part of this approach was to hirefarmers to grow switchgrass as well as to createa pilot biorefinery located in Vonore, TN. Rawmaterial transportation values to a pelletizationplant were assumed. Because no commercialswitchgrass pellet mill data were available, datafrom a survey of wood pelletization mills in the149US Southeast were used (Reed et al 2012). Primary data for the cultivation and harvest ofswitchgrass were collected by a survey of participating farmers (UTOBP 2010). In this study,growth and cultivation are used interchangeably.DataCultivation and harvest data were collected by asurvey sent to switchgrass farmers. The cropstands ranged from first year to mature, third year.Because inputs (primarily fertilizer treatments)decrease after stand establishment and yieldsincrease, the data were averaged and weightedacross a 10-yr period (the assumed stand rotation).From September to December 2010, 61 switchgrass farmers contracted with the UTBI werecontacted and sent a production data survey.The surveyed farmers operated switchgrassfarms primarily in the southeastern region ofTennessee (Vonore, TN). Another gate-to-gatelife cycle inventory (LCI) was conducted on thehardwood flooring residues pelletization processusing a similar survey method sent to operatingpellet mills in Alabama, Kentucky, Tennessee,Virginia, and West Virginia (Reed et al 2012).SimaPro LCA modeling software (Pré Consultants 2012) calculated the overall cradle-to-gateemissions of switchgrass pelletization using anetwork of related inventories associated withinputs for both the cultivation and harvest ofswitchgrass (resource harvesting) and the pelletization process (gate-to-gate). The US LCIDatabase (NREL 2012) was the main secondaryLCI data source.Wood pellet value was chosen because it isbased on regional commercial production of asimilar material. The procedures and report ofthis study follow the standards in ISO (2006a,2006b). The procedures and report also followthe research guidelines for LCI used by otherresearchers in the Consortium for Research onRenewable Industrial Materials (CORRIM) group(CORRIM 2010).The cradle-to-gate manufacture of switchgrass pellets comprises the following processes: cultivation
150WOOD AND FIBER SCIENCE, APRIL 2015, V. 47(2)and harvesting of switchgrass, transportation(switchgrass feedstock), storage, drying, sizereduction by hammer-milling, pelletizing, cooling,and storing (bagged or in bulk). This study didnot include bagging in the system boundary andevaluated the system as if the pellets were to beprepared for bulk storage.Unit Process and System BoundaryThe processes described in Fig 1 are the basicflows within the system boundary for the cradleto-gate LCI of the switchgrass pellet manufacturing model. The functional unit was 1 Mg(1 tonne ¼ 1000 kg) of switchgrass pellets(5% MC). The following describes each of themanufacturing processes:Cultivation and harvest. Switchgrass is grownas a perennial crop, with each stand lasting for10 yr before reestablishment. In the Southeast,cultivation and harvesting usually occurs duringmid to late fall. Initially, fields are plowed, fertilized, seeded, and treated with herbicides forweeds. No irrigation is used. During harvesting,the switchgrass is cut, field-dried, baled, andloaded on tractor-trailer trucks for transport.Farmers in our study used switchgrass farmingrecommendations from the UTBI (Garland 2007).Transportation. After baling, the switchgrassis loaded on diesel tractor-trailer trucks andFigure 1.transported to the pellet mill for storageand processing.Storage. In a projected model of a switchgrass pelleting facility, raw material would betransported by truck to the pellet mill. The rawmaterial would then be stored in a dry facility onsite. Inputs for raw material collection includediesel fuel and/or liquid propane gas for transportation and/or handling. No storage losseswere assumed.Drying. Raw materials for pelletizing usuallyrequire drying to about 10% MC. According toSanderson et al (2006) and Shinners et al (2010),initial moisture content of switchgrass at the timeof cutting ranges from 43 to 66%. Volatile organiccompounds emitted during drying were estimatedat 0.04 kg/Mg switchgrass (Eller et al 2011).Pelletization. Because of the inability to collect data for the pelletization process, this studytreated pelletization as a single process fromReed et al (2012). Pelletization includes sizereduction, pelletizing, and cooling processes.After the material is collected, it is broken downinto small, uniform particles ( 2 mm) using ahammer mill. The hammer mill is operated byelectric motors. Next, pellets ( 6 mm in diameterand 25 mm long) are extruded using machinerythat is similar to the equipment used to form feedpellets for the agriculture industry. Pelletizersuse large electric motors to extrude the pelletsthrough steel dies. High pressure ( 300 MPa)System description for the cradle-to-gate production of switchgrass pellets.
Bergman et al—SWITCHGRASS FUEL PELLETS CRADLE-TO-GATE LCAand temperatures ( 90 C) soften lignin andbind the switchgrass particles together to makeuniform and consistent pellets. Although noadhesives are required for this process, smallamounts of lubricants and water are sometimesadded to improve processing. Finally, the pelletsare hot when they emerge from the pelletizer.They are stored in a hopper and allowed to coolunder ambient conditions before further handling, transportation, and consumption.AssumptionsAssumptions and data collection and analysisfollowed the protocol defined in CORRIM (2010).Additional conditions included the following: All data from the switchgrass farmer survey were weight-averaged across a period of 10 yrto account for input and yield variations during the 10-yr life of the stands (Smeets et al2009; Daystar et al 2014). The input and yieldvalues reported for year 3 were assumed to bethe same for the subsequent 7 yr.The seeding rate (only for the first year) wasassumed to be 7.8 kg/ha (USDA NRCS PMP2009). The impacts associated with seeds comefrom the seed inputs used.Given that the machinery and storage facilities(from the farmers we surveyed) are one-timeconstructions and are used in various othercapacities, storage facilities, pellet facilities,and farm machinery construction were notconsidered in this study. Not including capitalgoods and infrastructure is consistent withLCI data from other energy sources includingnatural gas found in the US LCI Database(NREL 2012). Natural gas was used in thisstudy as a comparison with switchgrass pelletsfor energy use, enabling a direct comparison.Transportation of switchgrass at 20% MC fromfield to mill was based on an assumption of a73.9-km average haul distance.Switchgrass typically dries on the field frombetween 43 and 66 to 20% MC or less beforebeing baled and shipped to the plant for drying (Rinehart 2006; Sanderson et al 2006).Also, as the growing season progresses, the151moisture content decreases. Drying input(natural gas) of the switchgrass prior to pelletization was estimated by an expert in the areaof biomass processing (Follmer 2012) at872 MJ/Mg to dry switchgrass to 10% MC,thus requiring 27.09 m3 of natural gascombusted at 80% efficiency (FPL 2004). The pelletization process for dry switchgrasswas assumed to be the same as for pelletization of wood processing residues, as reportedby Reed et al (2012) except no wood residueswere burned for drying the incoming switchgrass. The process of pelletizing switchgrasswill change the density of the biomass rawmaterial and will evaporate some moisture.Therefore, it was assumed that 0.95 Mg ofoven-dry switchgrass is required to produce1 Mg of pellets with a final product moisturecontent of 5%. From previous literature, weassumed density for switchgrass pellets wouldbe 600 kg/m3 (Jannasch et al 2001). This study assumed 100% yield of pelletsfrom the raw material and that no raw material was lost as dust and that all poorly formedpellets or fine particles were recycled inthe system. However, Jannasch et al (2001)speculated that 95% yield might be more realistic, because of the loss of raw material during processing.Life Cycle Impact Assessment MethodLCI assessment was performed using the tool forthe decrease and assessment of chemical andother environmental impacts (TRACI 2) (Bare2011). TRACI is a midpoint-oriented methodology developed by the US Environmental Protection Agency specifically for the US. Nine impactcategories were examined, including globalwarming potential (GWP [kg CO2-eq]), acidification potential (Hþ moles-eq), carcinogenics(kg benzene-eq), noncarcinogenics (kg tolueneeq), respiratory effects (PM 2.5-eq), eutrophication potential (kg N-eq), ozone depletion (kgchlorofluorocarbons-11-eq), ecotoxicity (2,4-Deq), and smog potential (kg NOx-eq). Carcinogenics and noncarcinogenics effects are humanhealth impacts (Huijbregts et al 2005).
152WOOD AND FIBER SCIENCE, APRIL 2015, V. 47(2)Life Cycle InventoryOf the 61 farmers surveyed, 12 (19%) respondedwith complete data in terms of switchgrass production, seed rate, fuel use, and herbicide andpesticide inputs. The survey was sent out in2010 and represents cultivation and harvest datafor 2008, 2009, and 2010. Usable responseswere collected from 12 farmers with data on152 ha, or about 7% of the 2090 ha in switchgrass production. The survey responses for woodpellet manufacturers represented 2009 productiondata from approximately 25% of the total number of operating mills in the Southeast. The onlyavailable wood pellet production data estimatedtotal production in the Southeast region in 2008at 591.8 kt (Spelter and Toth 2009). Total production of the responding wood pellet mills was303.9 kt of pellets per year or about 51%.The switchgrass yield data collected in our survey (13.9 Mg/ha) were consistent with publisheddata (Jannasch et al 2001; Mooney et al 2009),but reported inputs (eg electricity) were less.However, the reported data on the pelletizationprocess were consistent with energy consumptionsreported in other studies (Thek and Obernberger2004; Mani et al 2006; Hagberg et al 2009;Sokhansanj et al 2009; Zhang et al 2010; Sjolieand Solberg 2011; Uasuf and Becker 2011;Katers et al 2012; Pa et al 2012). The weightaveraged electrical usage of 145.5 kWh/Mgreported per functional unit of wood pellets usedin this study (Reed et al 2012) was significantlymore than values estimated in some publicationsfor switchgrass pelletization (Smeets et al 2009).The inputs per functional unit (1.0 Mg) werecalculated based on an average switchgrass yieldof 13.9 Mg/ha per year (oven-dry, 0% MC).Variation in the reported inputs was great, inpart, because of differences in the ages of theswitchgrass stands (see coefficient of variation,Table 1). Of the material inputs for the cultivation and harvest of switchgrass, the most significant was nitrogen fertilizer. Other inputs includeddiesel, phosphorous fertilizer, pesticides, herbicides, and surfactant. The most significant inputfor pellet-making operations in the Southeastwas electricity. Other fuels used for equipment(ie tractors, trucks, forklifts) included diesel fueland liquid petroleum gas. Other raw materialinputs used in the manufacture of wood pelletsare water, oil, and grease. Water is used to adjustthe moisture content, and oil and grease are usedfor lubrication during the pelletizing process.Pelletization primarily has energy and switchgrass inputs and only one output—switchgrasspellets. Finished pellets contain about 5% MC.Therefore, the switchgrass input is only 0.95 Mgof oven-dry switchgrass to produce 1.0 Mg ofswitchgrass pellets (Table 2). Feedstock is driedbefore arriving at the pellet mill.Pelletization does not create a solid waste stream.All residues are recycled in the pelletizationprocess, and airborne particulate emissions(dust) are assumed to be insignificant and thuswere not included in the analysis. On-site pelletizing air emissions, water effluents, and solidwaste generation were insignificant. The emission data in this study (Tables 3 and 4) wereTable 1. Cradle-to-farm-gate inputs for 1.0 Mg of switchgrass (oven-dry basis) in the US Southeast.InputsUnitsAverage valueaCoefficient of variation (%)bDiesel (tractor use)Nitr
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