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1 Applied Energy 87 (2010) Contents lists vilble t ScienceDirect Applied Energy journl homepge: Evlution of energy efficiency of vrious biogs production nd utiliztion pthwys Mrtin Pöschl, *, Shne Wrd, Philip Owende,b Chrles Prsons Energy Reserch Progrmme, Bioresources Reserch Centre, School of Agriculture, Food Science nd Veterinry Medicine, University College Dublin, Belfield, Dublin 4, Irelnd b School of Informtics nd Engineering, Institute of Technology Blnchrdstown, Blnchrdstown Rod North, Dublin 15, Irelnd rticle info bstrct Article history: Received 30 November 2009 Received in revised form 6 April 2010 Accepted 10 My 2010 Avilble online 2 June 2010 Keywords: Energy performnce Biogs project Primry energy input Biogs conversion pthwy Energy input output rtio The energy efficiency of different biogs systems, including single nd co-digestion of multiple feedstock, different biogs utiliztion pthwys, nd wste-strem mngement strtegies ws evluted. The input dt were derived from ssessment of existing biogs systems, present knowledge on nerobic digestion process mngement nd technologies for biogs system operting conditions in Germny. The energy blnce ws evluted s Primry Energy Input to Output (PEIO) rtio, to ssess the process energy efficiency, hence, the potentil sustinbility. Results indicte tht the PEIO correspond to % nd % for single feedstock digestion nd feedstock co-digestion, respectively. Energy blnce ws ssessed to be negtive for feedstock trnsporttion distnces in excess of 22 km nd 425 km for cttle mnure nd for Municipl Solid Wste, respectively, which defines the opertionl limits for respective feedstock trnsporttion. Energy input ws highly influenced by the chrcteristics of feedstock used. For exmple, griculturl wste, in most prt, did not require pre-tretment. Energy crop feedstock required the respect cultivtion energy inputs, nd processing of industril wste strems included energydemnding pre-tretment processes to meet stipulted hygiene stndrds. Energy blnce depended on biogs yield, the utiliztion efficiency, nd energy vlue of intended fossil fuel substitution. For exmple, obtined results suggests tht, wheres the upgrding of biogs to biomethne for injection into nturl gs network potentilly incresed the primry energy input for biogs utiliztion by up to 100%; the energy efficiency of the biogs system improved by up to 65% when nturl gs ws substituted insted of electricity. It ws lso found tht, system energy efficiency could be further enhnced by % through recovery of residul biogs from enclosed digestte storge units. Overll, this study provides bses for more detiled ssessment of environmentl comptibility of energy efficiency pthwys in biogs production nd utiliztion, including mngement of spent digestte. Ó 2010 Elsevier Ltd. All rights reserved. Contents 1. Introduction Mteril nd methods Bckground dt Dt nlysis Description of scenrios nlysed Feedstock digestion scenrios Biogs utiliztion scenrios Wste strem scenrios Rtionlistion of study ssumptions Determintion of biogs yield by feedstock options Considertions in feedstock supply logistics Energy input to feedstock trnsporttion Biogs plnt opertion * Corresponding uthor. Tel.: E-mil ddress: mrtin.poeschl@web.de (M. Pöschl) /$ - see front mtter Ó 2010 Elsevier Ltd. All rights reserved. doi: /j.penergy

2 3306 M. Pöschl et l. / Applied Energy 87 (2010) Biogs utiliztion Electricity genertion Combined Het nd Power (CHP) genertion Fuel cell technology Stirling engines Micro gs turbines Het genertion Process het demnd of biogs system Het utiliztion vi district heting network Orgnic Rnkine Cycle process Other res for biogs utiliztion Upgrding of biogs Processing nd hndling of digestte Results nd discussion Considertion of the single feedstock digestion scenrio Considertion of feedstock co-digestion scenrio Considertion of biogs utiliztion scenrio Considertion of wste strem scenrio Conclusions Acknowledgement References Introduction The Europen Energy Policy is underpinned by: need to minimise exposure to voltility of fossil fuel prices; need for reduction of greenhouse gs (GHG) emissions by using less, clener, nd loclly produced energy, including the energy recovery from wste, nd need for more competitive energy mrkets to stimulte technology innovtion nd jobs [1]. In Germny, the focus is to increse renewble energy utiliztion from the current 9.1% to 20% by Consequently, the government developed n integrted energy nd climte progrm for structured GHG reduction, in which 25 30% of electricity nd 14% of het requirements is to be generted from renewble sources [2]. Biogs production by nerobic digestion (AD) of orgnic wste is key to meeting these trgets [3]. The estimted biogs production potentil, bsed on feedstock resource vilbility in Germny is 417 PJ per nnum [4]. Over 80% of the estimted potentil derives from griculturl resources, including frm wste, crop residues, nd dedicted energy crops (96.5, 13.7, nd 236 PJ per nnum, respectively). To relize this potentil, enhncement of incentives nd minimistion of brriers to expnsion of biogs production will be required, including: (i) simplifying the procedures in biogs plnt implementtion process; (ii) enhnced Reserch nd Development on feedstock co-digestion, especilly of gri-food industry wste nd Municipl Solid Wste (MSW) strems; nd (iii) enhncement of biogs utiliztion through provision of incentives to biogs plnts, e.g., economic feed-in triffs nd ccess to electricity nd gs grid infrstructure [5]. Biogs is rgubly more verstile renewble energy source (cf. wind nd solr energy), due to its determinte energy vlue nd ese of storge, hence, potentil utiliztion is significntly independent of fctors such s geogrphicl loction nd seson [6]. It cn be used directly for heting nd electricity genertion, nd s substitute for fossil fuel pplictions, e.g., trnsport fuel [4,7,8]. The potentil utiliztion of the digestte [9] s fertilizer cn lso reduce dependence on energy intensive minerl fertilizers, to further mitigte GHG emissions [10]. Since the ctivtion of the bn on lndfilling of orgnic wste in Germny [11], the AD process provides vible wste mngement option [12], but sustinble biogs utiliztion requires mintennce of positive life-cycle energy blnce. Anlyses of energy blnce in the life-cycle of biogs systems tht hve been reported to-dte often lck bses for comprison, due to vrying ccounting system nd boundries [13]. In prticulr, mny studies on energy blnce hve focused on specific rw mteril [14 22], specific biogs systems [6,16,18,23 25], different wste mngement strtegies [26 28], nd on specific utiliztion options for biogs [29 31]. To the uthors knowledge, only the study by Berglund nd Börjesson [13] hs ddressed the entire life-cycle of different biogs systems. However, Berglund nd Börjesson [13] identified the min fctors ffecting the energy input/ output rtio for biogs systems, but did not ttempt to correlte these to the primry energy output. None of the nlyses reviewed hve coupled multiple feedstock scenrios to vible energy conversion pthwys to ssess: (i) impct of plnt loctions to minimise GHG emissions through reduced fossil fuel usge nd elimintion of existing technicl nd policy obstcles [5]; (ii) potentil for integrted efficiency enhncement for relibility nd to minimise cost; nd (iii) overll system sustinbility. The objective of this study ws to evlute the impct of feedstock (single nd co-digested) nd process chins (production, conversion nd utiliztion) on energy blnce of biogs systems. It ws to evlute ny ltent fctors determining the Primry Energy Input Output (PEIO) rtio, which could be used for further efficiency enhncement. 2. Mteril nd methods Fig. 1 shows the study boundry, encompssing; feedstock resources, hrvesting nd trnsport, biogs plnt opertion, biogs-to-energy conversion technologies, nd digestte hndling. Anlyses were bsed on literture dt relevnt to conditions in Germny Bckground dt The nlyses considered the three min feedstock mteril flows, including, griculturl wste, energy crops, nd Municipl Solid Wste (MSW) nd residues from food industry [32,33]. The number of biogs plnts in Germny hs incresed by lmost 2000 in the lst 5 yers, with n verge output of 500 kw el. Smll-scle biogs systems (<500 kw el instlled cpcity) re designted to hndle predominntly griculturl wste nd energy crop feedstock [34] with loclized energy utiliztion, but with option for electricity feed-in to the ntionl grid. Lrge-scle biogs

3 M. Pöschl et l. / Applied Energy 87 (2010) Nomenclture AD CO CH 4 CHP EEG GHG GJ PE K 2 O kw el kw th kw h el kw h th MCFC MJ PE nerobic digestion lime methne Combined Het nd Power System dry mtter content, dry mss Renewble Energy Sources Act greenhouse gs gigjoule of primry energy source potssium oxide kilowtt electricity lod kilowtt therml lod kilowtt hour electricity lod kilowtt hour therml lod Molten Crbonte Fuel Cell megjoule of primry energy source MP MSW MW h N Nm 3 NO x ORC P 2 O 5 PE PEIO PJ t t t VS t km megpscl Municipl Solid Wste megwtt hour nitrogen norml cubic metre (biogs yield) nitrogen oxides Orgnic Rnkine Cycle process phosphte primry energy Primry Energy Input Output rtio petjoule tonne of fresh mss tonne of feedstock dry mtter tonne of voltile solids tonne kilometre plnts (P500 kw el instlled cpcity) co-digest griculturl feedstock with industril wste nd MSW, with cpbility to feed-in biomethne to the nturl gs grid. Lrge-scle biogs plnts typiclly produce more thn 1.8 million m 3 of biogs per nnum, with feedstock hndling cpcity of c. 20,000 tonnes per nnum, s distinct from smll-scle biogs plnts with typiclly up to 10,000 tonnes per nnum. Tble 1 summrizes the primry energy inputs to minerl bsed resources considered. Primry energy refers to energy tht hs not undergone ny conversion or trnsformtion process. Process nlysis for griculturl nd MSW wste strems nd food industry residues considered the energy inputs for feedstock collection, trnsport nd pre-tretment. Anlyses for energy crops considered the entire supply chin from plnting nd cultivtion, through hrvesting nd trnsport processes. Cultivtion ws on rble lnd with no conflict with food nd/or fodder production, nd therefore opportunity cost of lnd could be ignored [35,36]. Options in biogs utiliztion included; upgrde to biomethne for trnsport fuel or fuel cell technology, nd conversion technologies including, CHP, Stirling engine, Orgnic Rnkine Cycle, nd Micro gs turbine. Digestte disposl, nd tretment nd hndling options such s seprtion, composting or drying the solid mtter to ese trnsporttion were lso considered [32,37] Dt nlysis Energy blnce ws evluted s rtio of Primry Energy Input to Output (PEIO). Energy input ws evluted s the sum of energy inputs to; crop cultivtion nd feedstock pre-tretment, feedstock collection nd trnsport, biogs plnt opertion, biogs utiliztion nd digestte processing nd hndling. Energy output ws evluted s the sum of potentil energy conversion from the biogs yield from respective feedstocks. The lower the PEIO vlue, the higher the efficiency of the system chin, nd energy output is deemed negtive for PEIO exceeding unity. It ws ssumed tht the potentil electricity nd het genertion, nd biomethne nd digestte yields would substitute equivlent energy generted from specific fossil fuel mix nd crop production inputs in Tble 1. Primry energy input Animl wste, crop residues Recovery Trnsport Combined het nd power (CHP) Electricity Het Decentrlized biogs utiliztion ORC process Public electricity grid Centrlized biogs utiliztion Energy crops Cultivtion & hrvesting Biogs plnt: nerobic digestion Het Power Gs purifiction nd enrichment Nturl gs substitute Fuel cell technology CG vehicle fuel Electricity nd het Public gs network Fuel sttion Food residues, industry wste strems, MSW Collection & pre-tretment Digestte (Seprtion nd/ or drying) Spreding on fields Sewge plnt Composting System boundry Primry energyoutput Fig. 1. Study boundry showing mteril nd energy flows.

4 3308 M. Pöschl et l. / Applied Energy 87 (2010) Tble 1 Primry energy inputs to production of fossil fuel bsed resources considered in this study. Specific energy vlue Mteril/process Energy input Diesel fuel (MJ PE l 1 ) Production nd distribution 63 Electricity (MJ PE MJ 1 ) Electricity mix for Germny Nturl gs (MJ PE MJ 1 ) b Production nd distribution 45.6 Heting (MJ PE MJ 1 ) Biogs c 1.3 Nturl gs, heting oil nd hrd 44.2 col b Seeds (MJ PE kg 1 ) Grss Corn 9.58 Whet 3.48 Chemicl fertilizer Nitrogen N 53 (MJ PE kg 1 ) Phosphte P 2 O Potssium oxide K 2 O 7.7 Lime CO 0.7 Pesticides (MJ PE kg 1 ) Dt dopted from [15]. b Dt dopted from [83]. c Dt dopted from [13] Description of scenrios nlysed Relistic feedstock scenrios, biogs utiliztion scenrios, nd wste mngement scenrios were nlysed to determine the energy performnce of systems tht re commonly used in Germny [38]. Wet mesophilic two-stge AD process depicted in Fig. 2 ws ssumed. Trnsport distnce between biogs plnt nd origin of feedstock ws ssumed to be within 5 km, while digestte disposl ws within 10 km rdius. Defined bse cse scenrios were used to ssess the energy performnce of lterntive biogs systems Feedstock digestion scenrios Single feedstock digestion ws only pplicble to smll-scle biogs plnts, nd bse cse ws cttle mnure [38]. Considering the possible vritions in feedstock vilbility, coupled with the importnce of energy efficient trnsporttion, nd AD process considertions, single feedstock digestion is considered unsustinble for lrge-scle plnts. For exmple, sub-optiml composition of trce elements in single feedstock cn impede the AD process [39]. Similrly, rpid cidifiction of esily degrdble feedstock, e.g., food residues, my result in unsuitble conditions cpble of stlling the AD process [40]. Consequently, most biogs systems in Germny co-digest between three nd five feedstock [38]. Tble 2 represents severl co-digestion scenrios for smll (Bse SS, SS-1, SS-2) nd lrge-scle biogs plnts (Bse LS, LS-1, LS-2) investigted to determine their impcts on the PEIO nd the chrcteristic proportion of energy input to AD process steps. Biogs yield in codigestion ws ssumed to be 10% higher thn in single feedstock digestion (Fig. 3). Between 75% nd 80% of biogs plnts co-digest cttle mnure nd corn silge [38], therefore the combintion ws used s bse Steriliztion feedstock Heting digesters Orgnic feedstock Collection & pre-tretment Recovery Cultivtion & hrvesting Trnsport 5km Biogs plnt digester 1 Electricity input Het demnd Biogs plnt digester 2 CHP Electricity demnd Digestte Het Power Loding, trnsport 10 km Public grid Spreding on fields Fig. 2. Schemtic of biogs-to-chp process chin considered in single feedstock digestion nd feedstock co-digestion scenrios. Tble 2 Different co-digestion scenrios nlysed s typicl exmples of systems in opertion (Germny). AD feedstocks Smll-scle biogs systems (SS) Lrge-scle biogs systems (LS) Bse cse (Bse SS) b,c SS-1 d SS-2 e Bse cse (Bse LS) f LS-1 f LS-2 g Cttle mnure 55% 30% 40% 13% 25% Strw 5% Corn silge 45% 50% 35% Grss silge 10% 18% Whole whet plnt silge 10% 22% Municipl Solid Wste 14% 90% Food residues 20% 10% 6% Pomce 10% Slughterhouse wste (punch content) 15% 14% 4% Grese seprtor sludge 10% 49% Smll nd lrge-scle biogs systems typiclly hndle up to 10,000 nd t lest 20,000 tonnes feedstock per nnum, respectively. b Approximtely 75% of biogs systems in Germny use cttle mnure in co-digestion for biogs production with n verge proportion of 55% [38]; The mended EEG includes n liquid mnure bonus for using t lest 30% of liquid mnure for biogs production up to 150 kw el [41]. c Approximtely 80% of biogs systems in Germny use corn silge in co-digestion for biogs production [38], resulting from existing renewble energy resources incentive under the EEG. d Dt bsed on [84]. e Dt bsed on [85]. f Dt bsed on [86]. g Dt bsed on [87].

5 M. Pöschl et l. / Applied Energy 87 (2010) /t m 3 / yield (c), gs y biog ted b lcul Cl m = c 10% c = 0. 70m R ² = Mesured biogs yield (m), m 3 /t Fig. 3. Comprison of clculted biogs yield by feedtock proportion in feedstock co-digestion mix ginst mesured biogs yield for co-digestion in full-scle biogs plnts. The clculted vlues re bsed on dt from the Assocition of Technology nd Structures in Germny [49], wheres mesured vlues re bsed on relized biogs plnts in Germny [38,94] Biogs utiliztion scenrios Decentrlized power genertion with CHP units nd feed-in of excess cpcity to the ntionl grid is the most common biogs utiliztion pthwy [5,41]. In this study, dditionl biogs utiliztion pthwys were considered for their potentil impct on energy efficiency nd mitigtion of environmentl impcts by substituting different fossil fuels in ddition to electricity genertion (Tble 3). For exmple, utiliztion of biogs generted by smll-scle plnt, Bse SS-d, included CHP genertion nd secondry use of wste het in Orgnic Rnkine Cycle genertor to enhnce system efficiency Wste strem scenrios Different wste strem scenrios were used to ssess the potentil impct on PEIO (Tble 4). Spreding of untreted digestte on rble lnd ws only cceptble if the mount did not exceed soil nutrient limits (Bse LS). Seprtion of digestte into liquid nd solid frction to minimise hndling, nd lterntive drying/composting of solid frction for use s substitute for chemicl fertilizer were considered. Potentil impct of recovering residul biogs from digestte storge ws lso considered. 3. Rtionlistion of study ssumptions 3.1. Determintion of biogs yield by feedstock options cse for co-digestion in smll-scle plnts (Bse SS). Co-digestion of griculturl feedstock nd industril wste strems nd MSW is stndrd for lrge-scle biogs plnts hndling c. 20,000 tonnes per nnum, which ws bse feedstock for Bse LS scenrio (Tble 2). The feedstock nlysis considered redy vilbility nd suitbility for AD process (Tble 5). The unused feedstock potentil of cttle mnure in Germny is estimted t 90%, hence, it ws considered to be the most suitble bse feedstock [4,42]. Recovery of griculturl residues such s strw lso hs environmentl dvn- Tble 3 Different biogs utiliztion scenrios nlysed. Smll-scle biogs systems normlly utilize the biogs t source, nd in contrst to lrge-scle plnts which prefer trnsmission for wider ppliction. Biogs utiliztion pthwys Smll-scle plnts Lrge-scle plnts Bse SS b c d e f Bse LS A(U) B(U) C(U) D(U) E(U) CHP Externl het Cooling energy ORC Stirling engine Micro gs turbine Gs grid injection Trnsport fuel Fuel cell The CHP unit here is specificlly to provide het for biogs production nd upgrding processes. Tble 4 Different wste-strem mngement scenrios nlysed. Limittion on the mount of nutrients tht my be spred on griculturl lnd dicttes the vritions in digestte pretretment nd disposl technologies minly for lrge-scle biogs systems. Wste strem process Vritions nlysed Bse LS A(W) B(W) C(W) D(W) E(W) F(W) No tretment of digestte Seprtion by decnter technology Seprtion by screw-press technology Spreding of liquid frction Spreding of solid frction Drying of solid frction (by wste het of CHP) Composting of solid frction Substitute chemicl fertilizer Residul biogs utiliztion () (b) C(W) nd -D(W) re dditionlly clculted with residul biogs utiliztion by covered digestte storge re, showing influences on PEIO, which re mrked by initils () nd (b).

6 3310 M. Pöschl et l. / Applied Energy 87 (2010) Tble 5 Estimted biogs yield nd dry mtter () content of digestte by feedstock options dopted from [49], other dt evluted in this study. Feedstocks content feedstock b (%) content digestte c (%) tges in preventing nitrogen lekge into groundwter from decying process [43]. Cultivtion of energy crops such s corn, grss nd whet re subsidised therefore widely used in biogs production, nd ws ssumed to conform to good griculturl prctice [44,45]. Biogs yield is influenced by severl process conditions, nd vrition up to ±25% hve been recorded [46]. Process controlling prmeters such s temperture, retention time, volumetric loding, technology deployed (viz., continuous versus btch, single versus two stge digestion, slurry feeding cycle nd stirring technology, etc.), nd degree of pre-tretment of feedstock re importnt. Feedstock type is lso importnt, e.g., ft, proteins nd sugr contents hve high methne yield [47]. On the other hnd, inhibitors such s mmoni nd lipids in wste strems from bttoirs cn significntly derte yield [48]. Stge of energy crop hrvest, wether nd soil growth conditions my lso ffect biogs yield. Wheres theoreticl clcultions of biogs yield on the bsis of ft, crbohydrtes nd protein contents re strightforwrd [49], ccurte energy udit should be bsed on relistic dt tht cn be vlidted ginst performnce of full-scle plnts. Anlyses in this study for single feedstock digestion were bsed on biogs business plnning dt (Tble 5) compiled by the Assocition for Technology nd Structures in Agriculture (KTBL) in Germny [49]. Biogs yield in co-digestion ws estimted to be c. 10% higher tht yields from single feedstock digestion (Fig. 3) Considertions in feedstock supply logistics Tble 6 shows the energy input dt for crop cultivtion nd strw recovery. Strw recovery considered only griculturl mchinery fuel consumption during bling opertion. Energy crop feedstock considered lnd preprtion, seeding, fertilizer nd pesticide ppliction, nd hrvesting opertions. Utiliztion of industry wste, MSW, nd food wste considered pre-tretment nd/or steriliztion [50], including the removl of contrries like metl nd snd, nd finl pulping to fluidise the fibrous mtter [51]. Relted energy inputs re given in Tble Energy input to feedstock trnsporttion Biogs yield (m 3 t 1 ) Methne content (%) Energy content (GJ t 1 ) Cttle mnure Strw Corn silge Grss silge Whole whet plnt silge Municipl Solid Wste (MSW) Food residues Pomce Slughterhouse wste (punch content) Grese seprtor sludge Compositions of MSW is highly vrible nd cn significntly influence biogs yield [51]. b Fresh mss implies feedstock in consistence of origin, wheres dry mtter () is the result of fresh mss less the wter content. c Degrdtion of orgnic dry mtter content from feedstock results in different dry mtter content of digestte for single feedstock. Dt bsed on [88]. The primry energy inputs to feedstock collection nd trnsporttion considered: diesel fuel consumption per tonne per kilometre Tble 6 Energy inputs for energy crop cultivtion nd strw recovery [15,80]. Prmeter trnsported (tkm), nd customized refuse trucks with empty return trips, omitting ny secondry trnsport logistics [52]. Fuel consumption for the trnsporttion of griculturl feedstock ws determined by bnds of distnce covered, i.e., 62 km, km, nd >20 km (Tble 8). The quoted vlues cover loding processes. Fuel consumption ssocited with wste collection depends on fctors such s, collection re, distnce, trffic sitution, number of stops etc. The primry energy input for collection nd trnsporttion of MSW nd residues from food industry ws seprted into energy inputs for wste collection nd for trnsporttion between the collecting res nd designted biogs plnts. Solid orgnic wste ws source seprted. Tble 9 shows the energy input to collection nd trnsporttion of MSW; the collection component considered energy requirement for lifting bins nd compcting the wste [53,54]. Collection nd trnsporttion of industril orgnic wste strems (Tble 10) ssumed lesser utomted hndling opertions nd fewer number of stops compred to MSW [55] Biogs plnt opertion Corn silge Grss silge,b Whole whet plnt silge Strw Estimted yields (t h 1 ) Seeds (MJ PE t 1 ) Cultivtion/recovery (MJ PE t 1 ) Fertilizer Nitrte N (MJ PE t 1 ) Phosphte P (MJ PE t 1 ) Potssium oxide K 2 O (MJ PE t 1 ) Lime CO (MJ PE t 1 ) Pesticides (MJ PE t 1 ) Totl energy input (MJ PE t 1 ) Corn, grss nd whet losses in ensilge were ssumed to be 10% [14] nd strw losses in storge 5% [89]. b Permnent grsses: 4 hrvests per yer. Energy input to biogs plnt opertion vries with feedstock type nd deployed technology, but higher degree of utomtion in lrge-scle biogs plnts require reltively higher energy input [56]. Energy input to plnt opertion lso depends on; required stirring to mintin slurry homogeneity nd ese biogs relese, pump rting (liquid feedstock) nd conveying (solid feedstock), the feeding frequency, nd energy rting of uxiliry equipment. Esily bio-degrdble feedstock require higher feeding frequency to mintin stbility of the AD process [38]. Het demnd for AD in mesophilic temperture rnge (30 nd 37 C) depends on, mong other fctors: the integrity of therml lgging of the digesters; loction of digesters (underground or free- Tble 7 Energy inputs for pre-tretment nd steriliztion of feedstock. Feedstock Electricity (kw h el t 1 ) MSW Pre-tretment 60 Slughterhouse wste Steriliztion (20 min, (punch content) 133 C, 3 br) Food residues Pre-tretment nd steriliztion (1 h, 70 C) Het b (kw h th t 1 ) Averge electricity demnd of 150 kw h el per tonne dry mtter for pre-tretment of feedstock. Dt bsed on [55,76]. b 10% nd 15% of wste het from biogs-to-chp genertion ws used for steriliztion of food residues nd slughterhouse wste (punch content), respectively [50].

7 M. Pöschl et l. / Applied Energy 87 (2010) Tble 8 Energy inputs for trnsporttion of griculturl feedstock. Tble 10 Energy inputs for collection nd trnsport of industril orgnic wste strems. Feedstock % Energy input Truck lod (t) Collection (MJ t km 1 ) Food residues Pomce Slughterhouse wste (punch content) Grese seprtor sludge Feedstock Energy inputs by distnce bnd,b (MJ PE tkm 1 ) short distnce 6 2km mid-rnge distnce 2km6 20 km Cttle mnure Strw Corn silge Grss silge Whole whet plnt silge Using griculturl trnsporter consisting of trctor nd triler. b Dt bsed on [17,80]. Long distnce >20 km Tble 9 Energy input for collection nd trnsporttion of Municipl Solid Wste (MSW). Opertion Collection nd trnsport Distnce driven Energy input MJ t km 1 km MJ t 1 Collecting urbn res Trnsporttion urbn c Collecting rurl res b Trnsporttion rurl c Collection predominntly for lrge-scle biogs plnts; Dt bsed on [13,76,90]. Collection predominntly for smll-scle biogs plnts; Dt bsed on [13,76,90]. c Trnsporttion distnce between collection re nd designed biogs system ws ssumed to be 5 km. Dt bsed on [52]. Trnsporttion b (MJ t km 1 ) 50% efficiency, return trip empty. Collection for smll nd lrge-scle biogs plnts ws ssumed to be 40 km (rurl re) nd 15 km (urbn re), respectively. Dt bsed on [52]. b Outwrd trip full, return trip empty. Distnce between collecting re nd designed biogs system ws ssumed to be 5 km. Dt bsed on [52]. stnding); sesonl vritions in het demnd, nd feedstock type. Energy input for feedstock steriliztion (Tble 7) nd heting digesters ws serviced from the plnts own genertion cpcity (e.g. CHP). Process het in cse of biogs upgrding to biomethne ws provided from fossil fuels (Tble 1). The proportion of process het demnd for smll-scle biogs plnts ws ssumed to be higher thn for lrge-scle plnts (Tble 11), due to less efficient lgging of digesters [4]. Wet digestion process ws deployed for feedstock content of up to 12%, to llow for pumping nd stirring [39]. In the clcultion of primry energy input for individul feedstock, it ws ssumed tht 12% corresponded to 1 tonne of feedstock mixture. Therefore, 1 tonne of cttle mnure with 8% content ws ssumed to correspond to 0.7 tonnes of feedstock mixture, generting 0.7 tonnes of digestte, wheres 1 tonne of MSW (40% ) corresponded to 3.3 tonnes of mixture nd generting equivlent mount of digestte. The content of 12% ws chieved by dewtering feedstock using screw-press technology or dding wter, nd ccounting for 19 nd 32 MJ PE per tonne for smll nd lrge-scle biogs systems, respectively [57] Biogs utiliztion Electricity genertion Combined Het nd Power (CHP) genertion. Biogs-to-CHP represents the most common utiliztion in Germny. The generted electricity is fed into the ntionl grid, while the het my be consumed vi district heting networks, with typicl trnsmission losses presented in Tble 12. Prt of the generted het is used in the AD process control, nd for steriliztion of feedstock, if required. Efficiencies of CHP units nd respective energy inputs considered re presented in Tble Fuel cell technology. Deployment of fuel cell technology is t n dvnced development sttus in Germny [58]. Due to their high efficiency, low emissions nd multiple fuel possibility, the Molten Crbonte Fuel Cell (MCFC) is considered one of the most promising [59]. However, it is recognised tht relted investment costs re still high [60], therefore commercil deployment is yet to be chieved [58]. A het trnsmission distnce of 2 km ws ssumed s typicl for heting of green houses in horticulture (Tble 12). Energy input to MCFC ws equivlent to 2% of electricity generted [61], with electricl nd therml efficiencies of 50% nd 40%, respectively [59,62,63] Stirling engines. Stirling engines re non-fuel specific externl combustion engines tht require very little mintennce [64]. For units up to 100 kw el, electricl nd therml efficiencies re 24% nd 72%, respectively, which mkes het utiliztion ttrctive Tble 11 Energy inputs for biogs plnt opertion considered. Proportion of energy inputs for biogs plnt opertion, % Electricity demnd Het demnd Smll-scle biogs plnt () Energy input s proportion of electricity 4 25 produced in CHP (%) (b) Energy input s proportion of biogs produced (%) b Lrge-scle biogs plnt () Energy input s proportion of electricity produced in CHP (%) (b) Energy input s proportion of biogs produced (%) b Implementtion in cse of single feedstock digestion nd feedstock co-digestion scenrios. Dt bsed on [56,91]. b Implementtion in cse of upgrding biogs for substitution of nturl gs nd utiliztion s trnsporttion fuel. Dt bsed on [56,91]. Tble 12 Estimted het losses in district heting network; trnsmission losses depend on therml volume flow rte nd trnsmission distnce [49,61]. Estimted loss by trnsmission distnce, % 0.5 km 2 km 3 km 5 km Smll-scle biogs plnts (%) Lrge-scle biogs plnts (%)

8 3312 M. Pöschl et l. / Applied Energy 87 (2010) Tble 13 Efficiency nd electricity inputs for CHP genertion from biogs. Smll-scle biogs plnt (%) Lrge-scle biogs plnt (%) Efficiency nd electricity input for CHP genertion, % CHP electricl efficiency CHP therml efficiency,b Electricity input running CHP c Dt bsed on [39,92]. b Wheres the therml efficiency is lwys higher thn electricl efficiency for ll CHP units, the therml efficiency of lrge-scle units is typiclly lower thn of smll-scle units [39]. Up to 40% electricl efficiency hs been reported for CHP units >0.7 MW el electricl output [49,92]. c Electricity input is clculted from electricity produced by CHP [56]. [4,64,65]. Mximum het trnsmission distnce of 0.5 km ws ssumed (Tble 12). Biogs purifiction is not necessry; therefore, energy input of 2% of electricity generted ws ssumed [66 68] Micro gs turbines. Modulr micro gs turbine modules re rted t kw nd cn be redily combined in multiples to meet specific lod requirements [69]. Their lower combustion temperture results in lower NO x emissions [4,70]. The study ssumed electricl nd therml conversion efficiencies of 28% nd 54%, respectively [49], nd het trnsmission over 0.5 km (Tble 12). Since they require purified compressed biogs, energy input of 10% of the electricity generted ws ssumed [66,71] Het genertion Process het demnd of biogs system. Energy demnd for heting the digesters typiclly rnges between 20% nd 25% of totl het component from CHP genertion. Bsed on stipulted pretretment conditions (Tble 7), steriliztion of food residues nd slughterhouse wste consumed 10% nd 15% of totl het, respectively. Therefore, pproximtely 50% of generted het ws vilble for plnt processes including internl losses, nd the blnce ws vilble for externl use Het utiliztion vi district heting network. Externl het utiliztion vi district heting networks included spce heting (houses, niml stlls, greenhouses, nd public menities) ner to the generting biogs system. Het demnd will vry with seson, e.g., horticulture in greenhouses typiclly requires het for up to 5200 h yer [72], therefore, sustined use of 60% of the het vilble for externl use (30% of totl) ws ssumed. Energy input to trnsmission pumps equivlent to 2% of the useful het ws ssumed [61] Orgnic Rnkine Cycle process. The Orgnic Rnkine Cycle (ORC) process converts therml energy from low temperture het sources to electricity using orgnic fluids of high moleculr mss. Appliction is recommended for biogs plnts with output exceeding 300 kw th [73], nd where there is no demnd for het. The het-to-electricity conversion efficiency vries between 5% nd 17% [65,74], but verge efficiency of 12% ws ssumed. About 20% of therml energy from ssocited CHP genertion ws vilble for ORC process [61]. High nnul usge nd high system relibility re pre-requisites for economic opertion of ORC units [74]. Investment cost re pproximtely 60% of equivlent Stirling engine [75] Other res for biogs utiliztion. Absorption chillers provide dditionl possibility for utiliztion of wste het in combined het, power nd cooling (tri-genertion). Absorption chillers trnsform wste het to cooling energy with n efficiency of c. 70%, nd Tble 14 Energy input nd het demnd for upgrding of biogs. Energy input (MJ m 3 biogs) Electricity input 1.1 Het demnd 0.36 Compression to 1.6 MP b 0.18 Trnsmission of gs c 0.2 Compression to 20 MP d 0.47 typicl energy input is 70 kw h el per MW h of cooling energy lod [73]. It ws ssumed tht 40% of useful het is vilble for cooling, nd replces electricity generted from fossil fuel mix (Tble 1). The generted het from biogs systems cn lso be used for pplictions such s drying of griculturl produce nd the digestte. The ltter ws estimted to require MJ per tonne [49,76] Upgrding of biogs Upgrding of biogs to biomethne nd, where regultions permit, injecting into the nturl gs grid is n efficient wy of integrting the biogs into the energy sector [77,78]. Biomethne cn then be used s substitute for nturl gs nd s trnsporttion fuel [79]. Both utility re still unregulted in Germny, which mkes the determintion of long-term vibility unrelible [5]. Biomethne compression to 20 MP in pproprite cylinders for trnsport fuel consumes dditionl energy input [61], but it ws ssumed tht energy input per m 3 for cylinder trnsporttion to vending sttions ws negligible [7]. Tble 14 shows estimtes of relted energy inputs Processing nd hndling of digestte Energy input (MJ t km 1 ) Dt bsed on [61]. b Pressure required for injecting into nturl gs grid [29,49]. c Trnsmission distnce to the nturl gs network of 0.5 km ws ssumed [52]. d Pressure required for using on filling sttions [29,49]. Sustinble biogs utiliztion requires closed cycle of mtter, encompssing the complete recycling of digestte [5]. For lrgescle biogs plnts, limittion on the mount of nutrients tht my be spred on griculturl lnd [80] dicttes the mount of digestte tht cn be sfely recycled. Efficiency of digestte trnsporttion my be enhnced by on-site processing into chemicl fertilizer substitutes (decnter or screw-press ws used for sepr- Tble 15 Energy inputs in processing nd hndling of digestte. Activity Energy input (MJ PE t 1 ) Het demnd (MJ PE t 1 ) Seprtion Decnter 74.3 Screw-press 4.3 Liquid frction nd Loding 0.63 digestte b Trnsport 2.84 Spreding Solid frction Loding b 3.78 Trnsport b 3.15 Spreding b Drying c Composting d 510 Energy input (MJ PE tkm 1 ) Decnter (high speed centrifuge) nd screw-press technology, which seprte solid from liquid frction of digestte. Dt bsed on [37,49]. b Dt bsed on [80,93]. c Het lod for drying conveyor is met by output from CHP unit. Dt bsed on [49,76]. d Averge energy input for closed composting with min nd pre-composting creting commercil fertilizer. Dt bsed on [49,52].

9 M. Pöschl et l. / Applied Energy 87 (2010) tion). For exmple, seprtion of solid nd liquid phses cn reduce trnsporttion requirement by up to 60% nd nother 25% fter drying [23,76,81]. Digestte seprtion, loding, trnsport, nd spreding on rble lnd ccount for the primry energy input in processing nd hndling. In this study, typicl field mchine working width, pylods nd trvel distnce between biogs plnt nd field were considered [80]. Solid frction cn lso be composted for pproximtely 60 dys or lterntively dried with hot ir supplied from the system, then used s substitutes for chemicl fertilizer. Typicl digestte constituents from co-digestion of minly industry wste nd MSW re 4.6 kg N, 1.8 kg P 2 O 5 nd 3.8 kg K 2 O ( 5.7%) per tonne. The liquid frction cn re-circulted in the AD process, spred out to the fields, or disposed in sewge tretment plnt s ws ssumed in this study. Residul biogs in digestte tnks cn be up to 10% of biogs yield from the min AD process [4]; therefore, potentil recovery from digestte storge ws lso considered. Tble 15 outlines energy input to digestte processing nd hndling. 4. Results nd discussion 4.1. Considertion of the single feedstock digestion scenrio Fig. 4 shows tht the Primry Energy Input Output rtio (PEIO) in the single feedstock scenrio rnged from 10.5% to 64.0% depending on feedstock used. Feedstock with low energy density like cttle mnure (bse scenrio) required reltively higher primry energy input compred to primry energy output (Tble 5). Fig. 5 shows tht for single feedstock, biogs plnt opertion ws generlly the most energy-demnding process, nd proportion on energy input rnged from 7.0% (grese seprtor sludge) to 53.9% (strw). The disprities were ttributed to difference in content, e.g., 5% for grese seprtor sludge nd 86% for strw (Tble 5). Energy crop cultivtion nd feedstock pre-tretment ccounted for high proportion of the primry energy input. Fig. 5 shows tht for whole whet plnt silge, cultivtion process ws the most energy demnding (57.9% of totl primry energy input) in comprison to cttle mnure or grese seprtor sludge which do not require pre-tretment. It ws lso shown tht the difference in energy input ssocited with feedstock collection nd trnsporttion, nd digestte processing nd hndling cn be significnt. Such relte to trnsporttion efficiency, hence, the mount of digestte generted per tonne of feedstock. For exmple, Fig. 5 shows tht the proportion primry energy input for trnsport of cttle mnure feedstock (175 MJ PE t 1 ; 5.3%) is significntly lower thn for digestte processing nd hndling (1137 MJ PE t 1 ; 34.7%). This is explined by dditionl energy inputs for loding nd spreding of digestte. Feedstock collection nd trnsport of MSW nd slughterhouse wste ccounted for higher proportion of primry energy input (32.8% nd 26.5%, respectively) thn hndling of the respective digestte (5.0% nd 15.7%, respectively), due to the dditionl feedstock collection opertions. The dt shows tht the differences in totl primry energy input to feedstock supply logistics is influenced by origin nd chrcteristics of feedstock. For exmple, wheres feedstock from known wste strems such s cttle mnure, nd industril wste tht do not require steriliztion (e.g. pomce nd grese seprtor sludge),hence, only required energy input for collection nd trnsport; energy crops required dditionl input for cultivtion nd hrvesting of the crops (Fig. 5). Other feedstocks like MSW, food residues nd slughterhouse wste require energy-demnding pre-tretment processes before they cn be used for biogs production. Fig. 6 shows tht the PEIO vries considerble with trnsporttion distnce for feedstock nd digestte. It shows tht the trnsporttion distnces for grese seprtor sludge nd cttle mnure to trigger negtive energy blnce were less thn 17 nd 22 km, respectively. For cttle mnure, the low vlue ws ttributed to the lower specific biogs yield (297 m 3 t 1 ) combined with expected low trnsporttion efficiency due to low content of digestte (Tble 5). The high biogs yield from grese seprtor sludge would be expected to llow for efficient trnsporttion nd therefore cpble of mintining positive energy blnce over longer distnces (Tble 5). However, it ws observed tht energy input to feedstock collection nd trnsport, nd digestte processing Primry energy input/output rtio (%) 70% 60% 50% 40% 30% 20% 64.0% 60.4% 57.1% 53.7% 52.2% 46.6% Digestte processing nd hndling Biogs utiliztion Biogs plnt opertion Feedstock collection & trnsport Energy crop cultivtion & feedstock pre-tretment 36.7% 31.8% 25.6% 10% 10.5% 0% Cttle mnure (bse cse) Slughter house wste (punch content) Food residues Municipl Solid Wste Grese seprtor sludge Grss silge Pomce Whole whet plnt silge Corn silge Strw Fig. 4. Primry Energy Input Output (PEIO) rtio in single feedstock digestion scenrio.

10 3314 M. Pöschl et l. / Applied Energy 87 (2010) Proportion on energy input (%) 100% 80% 60% 40% 20% Energy crop cultivtion & feedstock pre-tretment Feedstock collection & trnsport Biogs plnt opertion Biogs utiliztion Digestte processing nd hndling 34.7% 10.5% 18.4% 14.1% 13.7% 5.0% 19.4% 35.8% 15.7% 49.5% 1.7% 4.0% 3.1% 2.8% 14.6% 4.0% 3.9% 4.0% 24.2% 23.8% 30.4% 7.0% 32.5% 30.0% 4.5% 53.9% 1.9% 1.8% 32.8% 0.7% 4.4% 34.7% 7.0% 13.9% 53.0% 7.0% 26.5% 0% 5.3% Cttle Strw mnure (bse cse) 24.5% 40.7% 56.7% 57.9% 46.0% 32.7% 25.0% 23.3% Corn silge Grss silge Whole whet plnt silge Municipl Solid Wste Food residues Pomce Slughter house wste (punch content) Fig. 5. Proportion of energy input to process steps by single feedstock digestion scenrios. 42.8% Grese seprtor sludge nd hndling equted to 92.3% of totl energy input (Fig. 5). For slughter house wste, corn silge nd whole whet plnt silge, the primry energy blnce ws estimted to turn negtive for trnsporttion distnces in excess of 72, 345 nd 460 km, respectively. These could be ttributed to their reltively high biogs yield (Tble 5) coupled with more energy efficient trnsport regime, consuming only % of totl energy input (see Fig. 5). The outlined observtions indicte the importnce of locl sourcing of feedstock nd/or disposl of spent digestte. They lso underline importnce of diversified feedstocks, therefore necessity for co-digestion regimes Considertion of feedstock co-digestion scenrio Fig. 7 shows the PEIO for co-digestion scenrios nlysed bsed on relistic feedstock proportions described in Tble 2. The estimted PEIO rnged between % nd % for smll nd lrge-scle biogs systems, respectively. The spred in PEIO vlues were lower thn for single feedstocks ( %; Fig. 4), which suggests more stble processes. Fig. 8 shows tht in codigestion, biogs plnt opertion ws still the most energydemnding process. For Bse SS-1, 70% of the totl feedstock mixture ws derived from energy crops consuming 860 MJ PE t 1 (20.4%) for cultivtion process compred to 440 MJ PE t 1 for Bse SS (12.5%) cultivting only 45% corn silge of co-digestion mixture (Tble 2). The proportion of energy input towrds feedstock collection nd trnsport in Bse SS-2, co-digestion of griculturl wste with industril wste strems ws five times higher (20.8%) thn for Bse SS digesting cttle mnure with corn silge (4.1%; Fig. 8). This disprity could be ttributed to high feedstock proportion (Bse SS-2; 55%) delivered by more elborte collection nd trnsporttion regime from privte households nd industry (Tble 9 nd Tble 10) compred to hndling nd delivery of feedstocks primrily from gri- Cttle mnure (bse cse) Corn silge Whole whet plnt silge Food residues Slughter house wste (punch content) Strw Grss silge Municipl Solic Wste Pomce Grese seprtor sludge Primry energy input/output rtio (%) 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Trnsporttion distnce for feedstock/ digestte (km) Fig. 6. Primry Energy Input Output (PEIO) rtio of single feedstock digestion scenrio s function of the trnsporttion distnce. It ws ssumed tht digestte hs to be trnsported the sme distnce s feedstock.

11 M. Pöschl et l. / Applied Energy 87 (2010) Primry energy input/output rtio (%) 60% 50% 40% 30% 20% 10% 0% Energy crop cultivtion & feedstock pre-tretment Feedstock collection & trnsport Biogs plnt opertion Biogs utiliztion Digestte processing nd hndling 55.0% 48.6% 48.2% 45.6% 44.6% 34.1% Bse SS SS-1 SS-2 Bse LS LS-1 LS-2 Fig. 7. The influence of co-digestion of multiple feedstocks on Primry Energy Input Output (PEIO) rtio for smll nd lrge-scle biogs systems (primry energy output: Bse SS: 7.8 GJ PE t 1 nd Bse LS: 17.3 GJ PE t 1 ). Proportion on energy input (%) 100% 80% 60% 40% 20% 0% Energy crop cultivtion & feedstock pre-tretment Feedstock collection & trnsport Biogs plnt opertion Biogs utiliztion Digestte processing nd hndling 17.5% 12.8% 11.7% 31.6% 28.0% 25.0% 7.7% 7.4% 5.7% 6.0% 5.6% 9.2% 34.1% 43.1% 51.8% 43.2% 40.8% 45.9% 14.7% 2.8% 20.8% 2.2% 18.5% 4.1% 20.4% 22.8% 12.5% 12.9% 9.2% 32.1% Bse SS SS-1 SS-2 Bse LS LS-1 LS-2 Fig. 8. Proportion of energy input to process steps by designed feedstock co-digestion scenrios for smll nd lrge-scle biogs systems. culturl wste scenrios (Tble 8). For Bse SS-1, co-digestion of minly energy crops with high content (Tble 5) required dilution to chieve 12% of feedstock mixture, which resulted in higher volume of digestte to hndle (28%; 1200 MJ PE t 1 ) thn the originl feedstock digested (2.8%; 120 MJ PE t 1 )(Fig. 8). Fig. 8 shows tht energy input for feedstock pre-tretment in Bse LS ccounted for 9.2% (550 MJ PE t 1 ), wheres in 2 it ccounted for 32.1% (1450 MJ PE t 1 ). The difference could be ttributed to the higher proportion of feedstock designted for mndtory pre-tretment/steriliztion (Tble 7). In contrst, only to 38% of feedstock combintions in Bse LS scenrio hd mndtory pre-tretment/steriliztion requirement (Tble 2). The results lso showed tht more thn double the primry energy output is relized from co-digestion of industril wste strems in lrge-scle biogs plnts (Bse LS, 17.3 GJ PE t 1 ) compred to griculturl feedstock (Bse SS; 7.8 GJ PE t 1 ). Higher ft content in grese seprtor sludge nd food residues digested in Bse LS probbly ccounts for the high biogs yield recorded in Tble Considertion of biogs utiliztion scenrio Fig. 9 depicts the influences of biogs utiliztion (energy conversion) pthwys on the PEIO, considering only the bse cse scenrios defined in Tble 3 (Bse SS, Bse LS). Fig. 10 shows the proportion of energy input to process steps by designted utiliztion pthwys. Fig. 11 depicts the svings in primry energy for different utiliztion pthwys resulting from substituting different fossil fuels used in feedstock-to-biogs process (Tble 1). The energy output of co-digestion scenrios Bse SS nd Bse LS (Tble 2) substitute electricity, het, nturl gs nd trnsporttion fuel within different biogs utiliztion pthwys for smll-scle nd lrge-scle biogs systems, respectively (see Tble 5, Fig. 3, Section 3.5). The generted energy from biogs (electricity, het, biomethne) substitute fossil energy sources in the sme re, nd therefore, the primry energy inputs needed to produce fossil fuel bsed resources (Tble 1). Clcultion of the primry energy substitution of fossil fuels like nturl gs with biomethne ws bsed on their clorific vlues [82]. The results show tht the PEIO in biogs utiliztion pthwys for smll nd lrge-scle plnts typiclly rnged between % nd %, respectively (Fig. 9). The rnges of vrition rise from the difference in efficiency of the respective energy conversion systems nd substitution of different fossil fuels used in feedstock-to-biogs process (Tble 1; Fig. 11). The rnge of vrition in ech cse depicts the inherent potentil for enhncing efficiency in biogs utiliztion. Fig. 10 shows tht the proportion of energy input to biogs utiliztion rnged between 6.0% (Bse SS) nd 18.1% (Bse SS-f),

12 3316 M. Pöschl et l. / Applied Energy 87 (2010) % 45% 40% 35% 30% 25% 20% 15% 10% 5% 0% Energy crop cultivtion & feedstock pre-tretment Feedstock collection & trnsport Biogs plnt opertion Biogs utiliztion Digestte processing nd hndling 45.6% 43.7% 39.1% 34.1% 26.9% 12.0% 8.7% 6.2% 5.7% 6.0% 6.1% 4.1% 1.3% Bse SS Bse SS- Bse SS-b Bse SS-c Bse SS-d Bse SS-e Bse SS-f Bse LS A(U) B(U) C(U) D(U) E(U) Primry energy input/ output rtio (%) Fig. 9. The influence of different biogs utiliztion pthwys on Primry Energy Input Output (PEIO) rtio for smll nd lrge-scle biogs systems. Proportion of energy input (%) Energy crop cultivtion & feedstock pre-tretment Feedstock collection & trnsport Biogs plnt opertion Biogs utiliztion Digestte processing nd hndling 100% 0.8% 0.8% 12.8% 9.6% 12.8% 12.6% 90% 23.9% 15.2% 15.6% 31.6% 30.8% 30.4% 30.2% 27.6% 31.6% 7.7% 7.4% 9.1% 80% 30.8% 70% 12.1% 6.0% 8.4% 9.5% 10.1% 6.1% 18.1% 60% 51.8% 52.0% 51.1% 50% 74.7% 74.4% 40% 45.9% 47.0% 44.7% 44.2% 43.9% 45.8% 38.8% 40.0% 30% 20% 10% 0% 18.5% 18.5% 18.2% 4.1% 4.0% 3.9% 4.2% 3.9% 4.1% 3.6% 13.9% 1.2% 1.2% 12.5% 12.1% 12.0% 12.8% 11.9% 12.4% 10.9% 9.2% 8.0% 6.9% 8.0% 9.3% 9.1% Bse SS Bse SS- Bse SS-b Bse SS-c Bse SS-d Bse SS-e Bse SS-f Bse LS A(U) B(U) C(U) D(U) E(U) Fig. 10. Proportion of energy input to process steps by designted biogs utiliztion scenrios. depending on process energy requirements nd efficiency of different technologies. For exmple, energy conversion with Stirling engine coupled with het utiliztion in Bse SS-e scenrio required 215 MJ PE t 1 of totl energy input; wheres, the energy conversion with Micro gs turbine in Bse SS-f ws thrice the mount t 735 MJ PE t 1 (Fig. 10). These results suggest tht the most energy efficient conversion pthwy (lowest PEIO) for smll-scle biogs plnts is the Stirling engine with utiliztion of the generted het (Bse SS-e: 4.1%, Fig. 9). This observtion ws minly ttributed to the high therml efficiency of the Stirling engine (72%). The utiliztion of wste het for secondry electricity genertion with ORC process recorded only mrginl gin in PEIO (cf. Bse SS-d t 43.7% versus Bse SS t 45.6%). Therefore, the ORC technology my only be recommended for systems tht do not include het pplictions in the vicinity of biogs plnt (Fig. 9). Avilble dt lso suggests tht the most vible utiliztion pthwy for smllscle biogs systems compred to Bse SS is CHP genertion with externl het utiliztion (Bse SS-; 6.2%) t reltively short trnsmission distnce (pproximtely 2 km). Het my be coupled to provision of cooling energy (Bse SS-c) on demnd, which bers mrginl (0.5%) decrese in PEIO. Energy input to biogs utiliztion for lrge-scle biogs plnts rnged from 7.4% to 30.8% (Fig. 10). The svings in primry energy input by substitution of process fossil fuel (nturl gs) with portion of generted biogs incresed by fctor of nine (754,700 MJ t 1 ) in cse of A(U) ginst 87,200 MJ t 1 (Bse LS) substituting minly electricity produced from Germn electricity mix nd fossil fuel mix for internl process het (Section 3.5.2; Fig. 11). The use of biomethne s trnsporttion fuel (C(U)) represents n ttrctive utiliztion pthwy with PEIO of 8.7% (Fig. 9). However, the number of filling sttions in Germny nd vrying costs for trnsporttion of compressed gs re still limiting fctors [7]. In preprtion of biomethne for injection into nturl gs network (A(U)), the proportion of energy input for biogs plnt opertion incresed by 22.9% compred to bse cse with 51.8% (Fig. 10). This ws ttributed to digester heting demnd, nd feedstock steriliztion nd gs upgrde technology bsed on fossil fuels (Tble 1). With coupled smll-scle CHP unit (B(U)), where prt of wste het is used for heting the digester(s) insted of fossil fuels, the energy input for plnt opertion ws lmost hlved to 38.8% compred to A(U) (Fig. 10). Bsed on the outlined nlyses, the most energy efficient conversion pthwy for lrgescle biogs systems include: (i) upgrding of biogs specificlly for gs grid injection, but using smll-scle CHP to service process nd biogs upgrding energy lods (estimted PEIO 1.3%) nd (ii) fuel cell technology with het utiliztion (estimted PEIO 6.1%).