Assuring the sustainable production of biogas from anaerobic mono-digestion

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bioti be Coa r t i c l e i n f oArticle history:Received 14 November 2013Received in revised form5 March 20142008). The production and use of renewable energy may helpmitigate climate change and also reduce dependence on fossilsources (Cherubini and Strmman, 2011). In line with this, theEuropean energy policy has the target of increasing the share ofrenewable energy to 20% by 2020 (European Parliament, 2009).energy generationtralized source offuel which can bef a wide range ofagriculture, live-l., 2012; Bacenettiaste available inre (Holm-Nielsenental and socio-economic benets ranging from the improvement of the fertiliserquality, reduction of odour and pathogens and its valorisation asbiogas are associated to the anaerobic digestion process (Holm-Nielsen et al., 2009). In parallel to animal manure, special atten-tion is being paid on the use of energy crops as potential feedstockdue to their high content of volatile solids, which renders highbiogas yields (Jury et al., 2010). In terms of physical and chemicalcharacteristics, energy crops are more homogeneous than organicwastes (Panoutsou, 2007). Therefore, dedicated crops such as* Corresponding author. Tel.: 34 881816739.E-mail addresses:, sara.gonzalez@usc.esContents lists availabJournal of Clean.e lsJournal of Cleaner Production xxx (2014) 1e12(S. Gonzlez-Garca).1. IntroductionClimate change is the most imminent environmental issue theworld is facing today (Appels et al., 2011). There is a generalconsensus that global warming is mainly caused by greenhousegases (GHG) of anthropogenic origin (Appels et al., 2011). GHGemissions associated to energy are the most important (w80% ofthe total), being electricity and heat production the largest emittingsector, followed by transport (European Environmental Agency,The interest on the biogas production for biois increasing since it provides a clean and decenenergy from renewable feedstock. Biogas is a bioobtained from the anaerobic digestion (AD) oorganic feedstocks, mainly organic waste fromstock, industries and households (Iglinski et aet al., 2014). The largest source of organic wEurope for biogas production is animal manuet al., 2009). Several agricultural, environmAccepted 6 March 2014Available online xxxKeywords:Biogas productionDigestate managementEnvironmental proleMaize silagePig slurry 2014 Elsevier Ltd. All rights reserved.Please cite this article in press as: Lij, L., et aProduction (2014), b s t r a c tThis study aims to analyse the potential environmental benets and impacts associated to the anaerobicmono-digestion of two different substrates (pig slurry and maize silage). The Life Cycle Assessmentmethodology was applied in two full-scale Italian biogas plants (Plant A - pig slurry and Plant B - maizesilage) in order to calculate the environmental prole of both systems with the aim of identifying themost suitable alternative from an environmental point of view. The study also includes credits due toavoided processes such as electricity production from the grid and mineral fertilisation as well as theconventional management of pig slurry regarding Plant A.The main outcomes show the importance of the feedstock composition on the environmental per-formance of these systems. While the assessment of Plant A ended up in environmental benets in allimpact categories as a consequence of credits related to replaced processes, its capacity for bioenergyproduction was limited. On the contrary, the use of maize silage as substrate provided a larger productioncapacity but it was also associated to negative environmental impacts. In this system, the cultivation ofmaize showed up as the largest responsible of the environmental impacts, specically due to diesel fuelconsumption in agricultural activities as well as on-site emissions linked to the application of fertilisers.A sensitivity analysis proved that the environmental prole of these bioenergy systems could beimproved through surplus heat use as well as technological improvements such as the replacement ofthe traditional dehumidication unit by a chiller. 2014 Elsevier Ltd. All rights reserved.Department of Agricultural and Environmenta15782 Santiago de Compostela, Spainb l Sciences e Production, Landscape, Agroenergy, University of Milan, Milan, ItalyAssuring the sustainable production ofmono-digestionLuca Lij a, Sara Gonzlez-Garca a,*, Jacopo BacenetMara Teresa Moreira aaDepartment of Chemical Engineering, Institute of Technology, University of Santiago djournal homepage: wwwl., Assuring the sustainable pr/j.jclepro.2014.03.022gas from anaerobic, Marco Fiala b, Gumersindo Feijoo a,mpostela, Rua Lope Gomez de Marzoa s/n,le at ScienceDirecter Productionevier .com/locate/ jc leprooduction of biogas from anaerobic mono-digestion, Journal of CleanerLCI life cycle inventorynermaize, wheat, triticale and sugar beet are being largely cultivatedfor energy purposes.Moreover, biogas production involves the production of avaluable co-product such as digestate, a stream rich in nutrientswhich could be used as an organic fertiliser for crop cultivation insubstitution of mineral fertilisers (Holm-Nielsen et al., 2009). Theuse of digestate would also allow returning nutrients back to thesoil (Abubaker et al., 2012).As a result, many agricultural biogas plants have been built inEurope in order to produce electric and thermal energy, especiallyin Germany, Denmark, Austria, Sweden and Italy (Bacenetti et al.,2013; Holm-Nielsen et al., 2009). Focussing on Italy, strong publicincentives have been granted for electricity produced from biogas(Ministerio dello Sviluppo Economico, 2012). Accordingly, there arearound 1000 biogas plants in the Italian agricultural sector. Most ofthem are in the northern regions such as Lombardy, Emilia Roma-gna and Veneto, where the largest number of livestock farms arelocated (Bacenetti et al., 2014).Although biogas production arises as a clean and environmentalsafe alternative for energy production, it is important to quantifythe environmental impacts associated to this process. Life CycleAssessment (LCA) is an internationally accepted method to gaininsight into the environmental consequences of a product or sys-tem (ISO 14040, 2006). This methodology has been widely used toassess the environmental prole of bioenergy production systemsand numerous studies can be found in literature (Jury et al., 2010;Lansche and Mller, 2012). In these studies, bioenergy systemsFU functional unitSS1 subsystem 1SS2 subsystem 2SS3 subsystem 3SS4 subsystem 4CHP combined heat and powerCSTR continuous stirred tank reactorVS volatile solidsCC climate changeOD ozone depletionAbbreviationsAD anaerobic digestionL. Lij et al. / Journal of Clea2provide good opportunities to achieve environmental benetswhen fossil fuels are replaced or when they are compared withconventional waste management schemes (Brjesson andBerglund, 2007). However, it is interesting to highlight that theenvironmental performance of a bioenergy system from biogas isconsiderably affected by the feedstock considered, the nal use ofthe biogas and the management of the digestate (Poeschl et al.,2012a,b).Therefore, the aim of this study was to assess the environmentalperformance and energy requirement of two different biogas pro-duction systems based on the consideration of pig slurry and maizesilage as feedstock.2. Materials and methods2.1. MethodologyLCA is a methodological framework useful to determine theenvironmental impacts of a system, product or activity (ISO 14040,Please cite this article in press as: Lij, L., et al., Assuring the sustainable prProduction (2014), LCA features a high developed methodology, which includesthe emissions of pollutants and material and energy consumptionsfrom raw material acquisition, through the production and usephases to waste management.2.2. Goal and scope denitionAs mentioned, the goal of this study was to evaluate from acradle-to-gate approach the environmental proles of two differentbioenergy systems (biogas to electricity). To do so, two real Italianbiogas plants which feature anaerobic mono-digestion wereassessed in detail following the ISO standards (ISO 14040, 2006).Table 1 encompasses technical data related to the plants understudy including feedstock and electrical power capacity (kWe).Plant A is located in the district of Lodi and performs the anaerobicmono-digestion of pig slurry. Northern Italy is one of the mostimportant European regions for livestock production (in particularmilk cows and pigs) (Eurostat; Holm-Nielsen et al., 2009). Conse-quently, the interest on biogas plants with manure as feedstock isbased on the wide availability of pig slurry. Plant B is located in thedistrict of Pavia and uses maize silage as feedstock. This plant wasselected because maize is the most commonly energy crop used forbiogas production in Europe due to its high yield of dry matter perhectare and high potential of methane production (De Vries et al.,2012b; Dressler et al., 2012).The life cycle environmental impacts of both plants wereTA terrestrial acidicationFE freshwater eutrophicationME marine eutrophicationPOF photochemical oxidant formationALO agricultural land occupationFD fossil depletionAEP avoided electricity productionACM avoided conventional managementBC base caseAS alternative scenariosAS1 alternative scenario 1AS2 alternative scenario 2AS3 alternative scenario 3Production xxx (2014) 1e12determined by building a Life Cycle Inventory (LCI), that is, theidentication and quantication of all relevant inputs and outputsows of each system. Specic objectives included the identicationof the most critical stages (environmental hotspots) in both bio-energy systems in order to identify opportunities to attain envi-ronmental benets.2.3. Functional unitAccording to ISO standards, the functional unit (FU) is dened asthe main function of the system expressed in quantitative terms(ISO 14040, 2006). The main function of these bioenergy systems isthe anaerobic digestion of feedstock for biogas production in orderto cogenerate electricity and heat. Therefore, the FU chosen was 1 tof feedstock mixture fed to the digester.2.4. Description of systems under assessmentFig. 1 outlines the main processes considered within each bio-energy system. All processes involved in both bioenergy systemsoduction of biogas from anaerobic mono-digestion, Journal of Cleanerwere aggregated into four main subsystems: feedstock production(SS1), feedstock supply (SS2), bioenergy production (SS3) anddigestate management (SS4). Furthermore, the potential replacedprocesses associated with the deployment of these bioenergy sys-tems were taken into account.2.4.1. Subsystem 1: feedstock productionPig slurry is the mainwaste stream of pig breeding farms and itsproduction is unaffected by its valorisation in anaerobic digestion;therefore, its production was excluded from the system boundariesof Plant A (Fig. 1a).Concerning Plant B (Fig. 1b), maize is extensively cultivated forenergy purposes in this region provided that its demand for food/Table 1Technical data of plants under study.Plant A Plant BAD District Lodi PaviaFeedstock Pig slurry Maize silageHydraulic retention time (days) 30e40 25e35Organic loading rate (kg VS m3 day1) 0.74 3.34CHP Engine power (kWe) 250 520Electrical efciency (%) 35.7 37.0Thermal efciency (%) 51.0 47.1Fig. 1. Flowchart and system boundaries of the bioenergy systems under study. (a) Plant A;indicate replaced processes.L. Lij et al. / Journal of Cleaner Production xxx (2014) 1e12 3Please cite this article in press as: Lij, L., et al., Assuring the sustainable prProduction (2014), Plant B. Note that dotted boxes indicate the system boundaries whereas grey boxesoduction of biogas from anaerobic mono-digestion, Journal of Cleanerfeed is completely satised. Accordingly, the assessment of itsproduction included all agricultural activities from eld prepara-tion operations to chopping step.2.4.2. Subsystem 2: feedstock supplyThe supply of pig slurry comprised both the collection of pigslurry in the breeding farm and its delivery up to the biogas plant bymeans of a tractor. Since pig slurry is collected and delivered ondaily basis, its storage inside the plant was considered negligibleand derived emissions were excluded from the study. Concerningmaize, chopped maize is transported to the biogas plant by a lorryas well as biogas losses (up to 1.5%) caused by leakages in valves andpipe connections.2.4.4. Subsystem 4: digestate managementThis subsystem comprises the storage of digestate and itsapplication on agricultural land. In Plant B, a separation of digestateinto liquid and solid fraction is previously carried out by means of ascrew press separator. In this sense, the solid fraction is stored andapplied as organic fertiliser, while the liquid fraction is recirculatedto the digester. In both systems, the storage of digestate takes placein the biogas plants. Derived storage emissions were taken intoL. Lij et al. / Journal of Cleaner Production xxx (2014) 1e124and it is subsequently ensiled and stored. During storage, 2% of themaize is lost.In both systems, the production of the machinery and diesel fuelrequired for the operations was taken into account within thesystems boundaries as well as combustion emissions derived fromdiesel fuel consumption.2.4.3. Subsystem 3: bioenergy productionThis subsystem encompassed all inputs and outputs required forthe production of biogas by AD (such as the loading of feedstock tothe digester, the AD process itself and the biogas treatment) as wellas its conversion into bioenergy in a combined heat and powergeneration (CHP) engine.While pig slurry is pumped to the digesterthrough pipelines, maize silage is loaded by a tractor to the bottomof a hopper fromwhich it is fed into the digester by means a screwauger.In both plants, AD takes place as a single-stage digestion processin a continuous stirred tank reactor (CSTR), operated at 40 C bymeans of the circulation of hot water. The hydraulic retention timeand the organic loading rate, expressed in terms of Volatile Solids(VS), are different in each plant (Table 1). In Plant B a dilution ofmaize silage with the liquid fraction coming from the separation ofthe digestate is required. Therefore, 1 t of feedstock inside the di-gesters contains 269.5 kg of maize silage.As a result of AD, biogas and digestate are produced. Biogas isstored in a gasholder dome placed at the top of the digesters. Inboth plants, the biogas produced is ltered, dehumidied anddesulphurised with a diluted solution of sodium hydroxide (8%)before being burned. Dehumidication is carried out by means of atraditional refrigeration unit fed with electricity coming from thegrid.Biogas is used in the CHP for both electricity and heat genera-tion. The electricity produced is fed into the Italian national grid,while the heat produced is used to maintain the temperature of thedigesters. Surplus heat is emitted as a waste to the atmosphere.Inputs of diesel fuel, water, sodium hydroxide, lubricant oil, heatand electricity are also required and, consequently, their produc-tion was included within the systems boundaries. In addition, theproduction of capital goods and infrastructure of both plants wastaken into account. The estimation of emissions is calculated on thebasis of diesel fuel consumption and biogas combustion in the CHPTable 2Global inventory data (per 1 tonne of feedstock digested) for Subsystem 2.Inputs from TechnospherePlant A Plant BMaterials and fuelsChopped maize (from SS1) 275 kgPig slurry 1 tDiesel fuel 0.60 kg 0.12 kgTractor 30 g 9 gAgricultural tillage 1.1 kg 9 gTransportLorry 0.82 tkmPlease cite this article in press as: Lij, L., et al., Assuring the sustainable prProduction (2014), within the subsystem boundaries while leaching duringstorage was assumed negligible because tanks are sealed.The digestate could be used as a potential organic fertiliserbecause it contains active fertiliser ingredients such as ammoniumnitrate (NH4-N), triple superphosphate (P2O5) and potassium sul-phate (K2O) (Brjesson and Berglund, 2006; Poeschl et al., 2012a).Thus, its delivery and application as organic fertiliser was also takeninto account within the system boundaries. All inputs required,including the production and use of machinery and diesel fuel aswell as derived emissions from diesel fuel combustion and organicfertilisation, were considered.2.4.5. Avoided processes derived from the valorisation of streamsIt was assumed that electricity produced in both bioenergysystems could be sold to the Italian grid and may substitute anequivalent amount of electricity from the Italian electric prole.Therefore, the avoided electricity production was included withinthe system boundaries. Moreover, it was also considered that theutilization of pig slurry for AD substitutes the conventional man-agement of pig slurry, consisting of its storage, transport and landspreading (ISPRA, 2008). Furthermore, avoidedmineral fertilisationwas considered due to the use of the digestate from Plant A as wellas the digestate solid fraction from Plant B as organic fertilisers. Allcorresponding inputs and outputs ows for each process weretaken into consideration within the systems boundaries.Thus, a system expansion strategy was performed in this studyavoiding allocation procedures between the digestate, electricityand/or biogas.2.5. Life cycle inventoryIn order to better represent both real bioenergy systems, fore-ground data comprised real data coming from the plants understudy. Primary specic data concerning inputs (diesel fuel, feed-stock, water, electricity, heat, sodium hydroxide and lubricant oil)and outputs (biogas, digestate, heat and electricity) associated withfeedstock supply (SS2), bioenergy production (SS3) and digestatemanagement (SS4) subsystems were supplied by means of surveysand on-site measurements in both plants. A detailed description ofLCI data corresponding to SS2, SS3 and SS4 is presented inTables 2e4, respectively.Outputs to TechnospherePlant A Plant BProductsMaize silage 275 kgPig slurry 1 toduction of biogas from anaerobic mono-digestion, Journal of CleanerTable 3Global inventory data (per 1 tonne of feedstock digested) for Subsystem 3.Inputs from Technosphere Outputs to TechnospherePlant A Plant B Plant A Plant BMaterials and fuels Products and co-productsMaize silage (from SS2) 269 kg Raw digestate 0.97 m3 0.92 m3Pig slurry (from SS2) 1 t Electricity 28.6 kWh 92.2 kWhDigestate liquid fraction (from SS4) 0.73 m3NaOH solution 6 g 26 gLubricant oil 18 g 34 gDiesel fuel 0.15 kgTractor 20 gAgricultural tillage 15 gEnergyElectricity 3 kWh 6 kWhOutputs to EnvironmentPlant A Plant BEmissions to airL. Lij et al. / Journal of Cleaner Production xxx (2014) 1e12 5Concerning maize production (SS1), agricultural data such asagricultural activities and inputs (digestate, fertilisers, pesticides,water and diesel fuel) were taken from the literature (Gonzlez-Garca et al., 2013), where Italian maize plantations were assessedin detail. In the aforementioned study, eld emissions derived fromfertilisers application were taken into account as well as carbondioxide captured during maize growth. All these data is encom-passed in Table 5. In addition, inputs and outputs ows related withavoided pig slurry conventional management, electricity produc-tion and mineral fertilisation are summarized in Table 6.Concerning transport distances, pig slurry was considered to bedelivered for an average distance of 4 km while a distance of 3 kmwas considered for the transportation of maize and digestate. In thescenario for conventional management of pig slurry, a distance of3 kmwas assumed for its transportation from the farm to the eld.The minimum required period of storage for both pig slurry anddigestate is dened by law: 150 days.In the processes related with the application of pig slurry,digestate and mineral fertilisers, eld emissions such as ammonia,nitrous oxide, nitrogen, nitrate and phosphate, were also estimated.Nitrogen-based emissions were calculated with factors provided byBrentrup et al. (2000) and phosphate emissions to water wereestimated according to Rossier (1998). Emissions associated withTable 4Global inventory data (per 1 tonne of feedstock digested) for Subsystem 4.Inputs from TechnospherePlant A Plant BMaterials and fuelsRaw digestate 0.97 t 0.92 tDiesel fuel 0.13 kg 0.17 kgTractor 6 g 4 gAgricultural tillage 12 g 7 gTransportLorry 0.56 tkmEnergyElectricity 0.42 kWhPlease cite this article in press as: Lij, L., et al., Assuring the sustainable prProduction (2014), avoided conventional management of pig slurry (methane,ammonia and nitrous oxide) were calculated with factors providedby ISPRA (2008), assuming an average production of 10 kg of pigslurry per head and day (Fiala, 2012). Emissions derived from thecombustion of biogas in the CHP were taken from NationalEnvironmental Research Institute (2010).Biogas losses derived from the storage of digestate and nitrogen-based emissions were estimated according to De Vries et al.(2012b). The quantity of mineral fertilisers that are potentiallyreplaced by the digestate was calculated on the basis of digestatecomposition and fertiliser replacement values (De Vries et al.,2012a).Finally, background data regarding the production of all requiredinputs were taken from ecoinvent database. Inventory dataregarding production of chemicals such as sodium hydroxide andCarbon dioxide 26.2 kg 104 kgMethane 0.20 kg 0.72 kgCarbon monoxide 89 g 0.32 kgNitrogen oxides 58 g 0.21 kgNMVOC 3 g 10 gNitrous oxide 0.5 g 1.6 gWaste heat 9.9 kWh 53.2 kWhlubricant oil were taken from Althaus et al. (2007). Data corre-sponding to the production of electricity and bioenergy infrastruc-turewere taken fromDones et al. (2007) and Jungbluth et al. (2007)respectively. Finally, inventory data concerning transport activities,agricultural machinery and agrochemicals productions wereconsidered from Spiermann et al. (2007) and Nemecek and Kggi(2007) respectively. The Italian electricity mix used in the calcula-tions is primarily composed of non-renewable energies, such asOutputs to EnvironmentPlant A Plant BProducts and co-productsRaw digestate 0.97 tDigestate solid fraction 0.19 tEmissions to airCarbon dioxide 0.42 kg 2 gMethane 0.16 kg 0.7 gAmmonia 0.75 kg 0.13 kgNitrous oxide 62 g 35 gNitrogen oxides 4 g 20 gNitrogen 0.30 kg 0.17 kgEmissions to waterNitrate 5.8 kg 2.2 kgPhosphate 8 g 3 goduction of biogas from anaerobic mono-digestion, Journal of CleanerLife Cycle Impact Assessment was conducted using character-Table 7 includes the characterization results corresponding toIn Table 7 important differences between the two systems underassessment can be observed. The environmental burdens associ-ated with Plant A where pig slurry is mono-digested ended up inenvironmental benets in all impact categories. These results aredue to credits provided by replaced processes (electricity produc-tion, conventional management of pig slurry and mineral fertil-isation). On the contrary, the results attributed to Plant B wheremaize silage is considered for mono-digestion accounted fornegative environmental impacts for almost all categories selecteddespite the benecial effects associated with the carbon uptake bybiomass, avoided electricity production and substitution of mineralfertilisation. It is important to remark that the production of feed-stock (SS1) is only taken into account in the assessment of Plant B.This subsystem has a positive effect in CC mainly due to carbondioxide uptake, whereas for the remaining impact categories, thissubsystem greatly affects its environmental prole. Even more,Plant A includes environmental credits associated with pig slurrymanagement.Furthermore, additional differences can be observed betweenboth bioenergy systems. In terms of FU, more electricity is producedwith SS3 in Plant B are larger than the ones for Plant A since higherbioenergy production entails higher rates of inputs (such as NaOHTable 5Global inventory data (per 1 tonne of feedstock digested) for Subsystem 1 related toplant B.Inputs from Technosphere Outputs to TechnosphereMaterials and fuels Products and co-productsMaize seed 78 g Maize straw 275 kgDigestate 0.29 kgUrea 0.39 kgMetolachlor 20 gAtrazine 8 gDiesel 1.3 kgTractor 55 gAgricultural tillage 0.19 kgInputs from Environment Outputs to EnvironmentResource Emissions to airCarbon dioxide 159 kg Ammonia 0.24 kgWater 17.3 m3 Nitrous oxide 21 gNitrogen 97 gEmissions to waterPhosphate 2 gL. Lij et al. / Journal of Cleaner Production xxx (2014) 1e126the FU chosen (that is, 1 t of feedstock digested) for the two bio-energy systems under assessment.ization factors from ReCipe Midpoint methodology (Goedkoppet al., 2009). The following impact categories were considered inthe analysis: climate change (CC), ozone depletion (OD), terrestrialacidication (TA), freshwater eutrophication (FE), marine eutro-phication (ME), photochemical oxidant formation (POF), agricul-tural land occupation (ALO) and fossil depletion (FD).In the presented results, positive values are indicative of envi-ronmental burdens whereas negative values denote environmentalcredits or benets accrued from the uptake of carbon dioxide aswell as from avoided processes.3.1. Comparative analysisnatural gas (39%), oil (13%) andhard coal (13%). Considering imports,renewable energies account for 24% of the total (Dones et al., 2007).3. ResultsTable 6Global inventory data (per 1 tonne of feedstock digested) for avoided processes.Inputs from TechnospherePlant A Plant BAvoided materials and fuels$Avoided conventional managementDiesel fuel 0.65 kgTractor 36 gAgricultural tillage 1.1 kg$Avoided mineral fertilisationAmmonium nitrate 2.3 kg 0.40 kgTriple superphosphate 1.8 kg 0.74 kgPotassium sulphate 3.6 kg 0.73 kgDiesel 87 g 0.15 kgTractor 20 g 35 gAgricultural tillage 54 g 93 gAvoided energy$Avoided electricity productionElectricity 29 kWh 92 kWhPlease cite this article in press as: Lij, L., et al., Assuring the sustainable prProduction (2014),, lubricant oil, electricity and heat) and derived emissions(see Table 3).Since the digestate produced is separated into solid and liquidfractions in Plant B, more digestate can be applied to land fromPlant A (970 kg) than from Plant B (187 kg) (Table 4). Therefore, theenvironmental burdens associated to digestate management (SS4)in Plant A are higher than the ones from Plant B. In the same way,credits reached by Plant A due to avoided mineral fertilisation arelarger than the ones for Plant B.3.2. Detailed assessment of Plant AFig. 2a shows the relative contributions of the different sub-systems to the environmental prole of the bioenergy systempresent in Plant A.Outputs to EnvironmentPlant A Plant BAvoided emissions to air$Avoided conventional managementCarbon dioxide 5.6 kgMethane 2.2 kgAmmonia 0.99 kgNitrous oxide 60 gNitrogen 0.25 kg$Avoided mineral fertilisationAmmonia 55 g 9 gNitrous oxide 44 g 7 gNitrogen 0.20 kg 32gAvoided emissions to water$Avoided conventional managementNitrate 6.1 kgPhosphate 6 g$Avoided mineral fertilisationNitrate 2.9 kg 0.48 kgin Plant B (92.2 kWh) than in Plant A (28.6 kWh) (see Table 3) dueto the higher yield of biogas related to maize silage and the higherelectric efciency of Plant B. Accordingly, the credits achieved bythe avoided electricity production in Plant B are greater than theones in Plant A. Additionally, the environmental burdens relatedPhosphate 8 g 3 goduction of biogas from anaerobic mono-digestion, Journal of CleanerTable 7Characterization results corresponding with the digestion of 1 t of feedstock. (a) Plant A; (b) Plant B. Acronyms: AEPe avoided electricity production; ACMe avoided pig slurryconventional management; AMF e avoided mineral fertilisation; SS1 e subsystem 1; SS2 e subsystem 2; SS3 e subsystem 3; SS4 e subsystem 4.Category Unit Total AEP AMF ACM SS2 SS3 SS4(a)CC kg CO2 eq 73.5 16.6 47.8 75.7 6.2 31.3 23.1OD kg CFC-11 eq 3.1 106 1.5 106 2.1 106 4.8 107 6.4 107 1.9 107 9.6 108TA kg SO2 eq 0.94 0.073 0.34 2.4 0.032 0.043 1.8FE kg P eq 0.012 4.0 103 0.010 2.8 103 2.2 103 6.2 104 2.7 103ME kg N eq 0.77 2.5 103 0.69 1.5 1.8 103 2.6 103 1.4POF kg NMVOC 0.055 0.043 0.086 0.061 0.050 0.074 0.019ALO m2 a 0.32 0.084 0.49 0.38 0.38 0.20 0.026FD kg oil eq 8.9 5.0 5.7 1.2 2.0 0.65 0.22Category Unit Total AEP AMF SS1 SS2 SS3 SS4(b)CC kg CO2 eq 15.3 53.6 7.9 94.2 0.77 128 11.2OD kg CFC-11 eq 2.7 106 4.7 106 4.4 107 6.9 106 1.2 107 7.5 107 5.0 108TA kg SO2 eq 1.4 0.23 0.073 0.85 4.9 103 0.15 0.34FE kg P eq 0.015 0.013 3.6 103 0.028 8.7 105 2.4 103 0.001ME kg N eq 0.44 8.1 103 3.9 103 0.039 3.0 104 9.5 103 0.51POF kg NMVOC 0.44 0.14 0.019 0.29 0.008 0.27 0.023ALO m2 a 23.2 0.27 0.11 22.6 0.059 0.86 0.013FD kg oil eq 7.2 16.1 1.3 21.6 0.28 2.5 0.14Fig. 2. a) Relative contributions from processes involved in bioenergy system related with Plant A; b) distribution of contributions from processes involved in Plant A excludingavoided processes. Acronyms: SS2 e feedstock supply; SS3 e bioenergy production; SS4 e digestate management; AEP e avoided electricity production; ACM e avoided pig slurryconventional management; AMF e avoided mineral fertilisation.L. Lij et al. / Journal of Cleaner Production xxx (2014) 1e12 7Please cite this article in press as: Lij, L., et al., Assuring the sustainable production of biogas from anaerobic mono-digestion, Journal of CleanerProduction (2014), environmental burdens of Plant A are related with pig slurrysupply (SS2), bioenergy production (SS3) and digestate manage-ment (SS4). In addition, this system includes credits derived fromreplaced processes such as electricity production, pig slurry con-ventional management and mineral fertilisation. As can be seen inFig. 2a, credits from replaced processes play a major role in thedenition of the system prole (between 58 and 81% of the totalburdens of this system).The potential substitution of electricity from the Italian electricprole due to the electricity produced from biogas (AEP) is signif-icant in impact categories such as OD, FE, POF and FD. This is mainlydue to the high ratio of non-renewable sources in the Italian electricprole.The conventional management of pig slurry (ACM) includes theuse of machinery required for the collection of the pig slurry in thefarm, its delivery and spreading. In addition, this process involvesemissions of methane, carbon dioxide, nitrogenous compounds andphosphate. This avoided process has remarkable contributions inall categories selected, especially in terms of CC, TA and ME. Con-cerning CC, the major inuence is related to methane emissionsfollowed by emissions of nitrous oxide and carbon dioxide. In thecase of TA, the total contribution is due to emissions of ammonia.Finally, emissions of nitrate and phosphate are responsible of thediesel consumed and the consequential combustion emissions. Itimplies important contributions in terms of OD, FE, POF, ALO andFD, representing between 37% and 69% of the total impact in thissystem. Impact categories such as FE and ALO are inuenced by theproduction of the machinery required for collection and delivery ofthe pig slurry. In the case of OD and FD, the impact is caused by theproduction of the diesel required in the machinery. Finally, com-bustion emissions derived from diesel consumption highly affectPOF.In Plant A, electricity from the Italian grid is used in loadingoperations, in the digester and in the biogas treatment. The con-sumption of this electricity contributes to impact categories such asOD, FE and FD, with contributions between 7 and 17% of the totalimpact produced (Fig. 2b). The emissions derived from the com-bustion of the biogas produced in the CHP entail important con-tributions in terms of CC (48%) and POF (49%), mainly due toemissions of carbon dioxide (CC) and nitrogen oxides (POF).After storage, digestate is delivered for its application as anorganic fertiliser. The application of digestate results in emissions ofnitrogenous compounds and phosphate, with contributions be-tween 47% and 99% of the total impact produced in TA, FE and ME.3.3. Detailed assessment of Plant BL. Lij et al. / Journal of Cleaner Production xxx (2014) 1e128avoided impact in terms of ME.As mentioned previously, the use of the digestate as an organicfertiliser implies the reduction of mineral fertilisation. These avoi-ded environmental burdens have an outstanding inuence inalmost all impact categories, especially in terms of OD, FE, POF, ALOand FD, mainly due to the large amount of energy required for themanufacture of mineral fertilisers.Since the credits provided by the replaced processes havegreat inuence on the system prole, the results were analysedwith the aim of identifying the components of the system withgreater environmental impacts. Fig. 2b displays the relative con-tributions of each component to the global environmental im-pacts for the biogas system of Plant A. Note that percentages ofimpact given in this analysis are only related to total negativeloads of the system.Activities related to pig slurry collection and transport comprisethe production of the machinery required in this subsystem, theFig. 3. Relative contributions from processes involved in biogas system related with Plantproduction; SS4 e digestate management; AEP e avoided electricity production; AMF e avPlease cite this article in press as: Lij, L., et al., Assuring the sustainable prProduction (2014), 3 reports the relative contributions of the different sub-systems and replaced processes considered within the bioenergysystem related with Plant B.Environmental burdens associated with Plant B are related tomaize production (SS1), maize supply (SS2), bioenergy production(SS3) and digestate management (SS4) as well as credits providedby avoided electricity production and mineral fertilisation.As shown in Fig. 3, the cultivation of maize (SS1) was identiedas the most important contributor to the environmental prole.Fig. 4 displays the most important processes related with maizecultivation. In terms of CC, maize production caused a positive ef-fect as a result of the CO2 uptake during crop growth. With regardto other impact categories selected, maize production arises withcontributions between 6 and 94% of the total negative impactproduced. Within the different activities involved, the irrigationoperations were identied as the main contributor, mainly due toB. Acronyms: SS1 e feedstock production; SS2 e feedstock supply; SS3 e bioenergyoided mineral fertilisation.oduction of biogas from anaerobic mono-digestion, Journal of CleanerssesL. Lij et al. / Journal of Cleaner Production xxx (2014) 1e12 9energy requirements. Field emissions derived from the applicationof digestate and urea (essentially ammonia) considerablycontribute to TA (70%) and ME (57%). Land occupation associated tocrop cultivation has an important effect in terms of ALO (72%).As depicted in Fig. 3, the production of bioenergy (SS3) entailsremarkable impacts along the system under assessment. Inparticular, this subsystem specially contributes in categories suchas CC (43%), TA (9%) and POF (36%), mainly due to emissions derivedfrom the combustion of the biogas in the CHP. Moreover, thedigestate management (SS4) inuences impact categories such asTA (20%) and ME (75%) due to its application as fertiliser.Credits provided by mineral fertilisation play a minor rolebecause of the small amount of the digestate applied. Nevertheless,the avoided electricity from the Italian national grid (AEP) con-tributes to reduce the environmental impacts produced by thissubsystem.4. DiscussionFig. 4. Relative contributions of proceBioenergy systems can achieve GHG emission savings whencompared to conventional fossil reference systems (Brjesson andBerglund, 2007). However, for other impact categories such as TA,FE andME,most bioenergy systems lead to increased impacts whencompared to fossil reference systems (Cherubini and Strmman,2011). This particularly applies to energy crops, where intensiveagricultural practices coupled to the use of N-based fertilisers cancause environmental concerns especially in impact categories suchas acidication and eutrophication, which are directly affected byN-based emissions (Cherubini and Strmman, 2011).Numerous LCA studies have analysed the environmental bene-ts and weaknesses of biogas production from different feedstocksTable 8Comparison between the obtained results with other studies.Category Unit Plant A Plant BFeedstock Pig slurry Maize silageCC kg CO2 eq 73.5 15.3TA kg SO2 eq 0.94 1.4FE kg P eq 0.01 0.01ME kg N eq 0.77 0.44a Note that these data correspond to anaerobic mono-digestion in small-scale biogasPlease cite this article in press as: Lij, L., et al., Assuring the sustainable prProduction (2014), and Berglund, 2007; Hartmann, 2006). The use ofdifferent input data, functional units, allocation methods, referencesystems, characterization factors and other assumptions impedesstraightforward comparisons of LCA bioenergy studies (Cherubiniand Strmman, 2011). In addition, uncertainties and use of spe-cic local factors for indirect effects may give rise towide variabilityof the nal results (Dressler et al., 2012).Poeschl et al. (2012a,b) investigated a wide variety of biogassystems including mono- and co-digestion scenarios with differentfeedstocks, biogas uses and digestate management processes.Regarding mono-digestion based scenarios in small-scaled biogasplants (as our case studies), the best results were obtained whenstraw (agricultural waste) and cattle manure (livestockwaste) wereconsidered as feedstock. The main features were the low environ-mental impact associated with feedstock production and the highbiogas yield in the case of straw, leading to corresponding substi-tution of electricity from the German electric prole. Moreover, thelargest environmental burdens were reached by the use of energyinvolved in maize cultivation process.crops as feedstock due to the higher fossil fuel consumption andfertilisers input associated with the agricultural step. Otherwise, DeVries et al. (2012b) assessed the environmental consequences ofthe anaerobic mono-digestion of pig slurry and its co-digestionwith different substrates. According to De Vries et al. (2012b), theanaerobic mono-digestion of pig slurry attained better environ-mental results compared to conventional manure management;however, it represents a limited source of energy comparedwith itsco-digestion with another feedstock.The results obtained in our study could be compared with thosereported by Poeschl et al. (2012b) and De Vries et al. (2012b)regarding similar biogas systems (Table 8). Specically, Poeschlet al. (2012b) analysed the use of cattle manure and maize silagePoeschl et al. (2012b),a De Vries et al. (2012b)Cattle manure Maize silage Pig slurry23.2 108 160.04 1.23 0.09e 0.01 0.010.02 1.18 0.03plants.oduction of biogas from anaerobic mono-digestion, Journal of Cleaner4.1. Sensitivity analysisA sensitivity analysis was conducted to assess the inuence ofimportant parameters within the environmental performance ofthe bioenergy systems under study. A comparison between thebase case (BC) and alternative scenarios (AS) proposed in bothplants was performed in order to verify the reliability of the resultsobtained as well as to assess possible improvement alternatives. Alternative scenario 1 (AS1): As explained above, surplus heatwas considered as a waste to the atmosphere in BC. In the rstalternative scenario a valorisation of this surplus heat by meansof an Organic Rankine Cycle (ORC) turbine (10 kW regardingPlant A and 30 kW regarding Plant B) was proposed. As a result,the electricity production of both bioenergy systems increased(2% in Plant A and 6% in Plant B) regarding BC. Alternative scenario 2 (AS2): In the second alternative scenario,the installation of an afterburner for the exhaust engine gaseswas proposed in both plants, reducing the emissions of carbonmonoxide to the atmosphere by 80%. Accordingly, part of thebiogas must be used in the afterburner and thus the nal elec-tricity production also decreased (4% in both plants) comparedto BC. Alternative scenario 3 (AS3): In the BC, the biogas treatmentner Production xxx (2014) 1e12and De Vries et al. (2012b) with the use of pig manure for anaerobicmono-digestion. It is essential to take into account important dif-ferences in the system boundaries dened for these studies, whichhad inuence on the environmental results. For example, Poeschlet al. (2012b) account credits due to avoided production of chem-ical fertilisers; but they did not consider the whole fertilisationprocess. Moreover, they neither consider credits due to avoidedmanure conventional management. Furthermore, despite De Vrieset al. (2012b) considered avoided management of pig slurry; theyalso considered land use change emissions and the production ofthe substitutes for the initial use of the co-substrates.In terms of CC, the best results regarding manure mono-digestion were attained by Plant A due to credits accrued by avoi-ded pig slurry conventional management. De Vries et al. (2012b)considered these credits but they also account burdens due toland use change emissions. Regardingmaize silagemono-digestion,Poeschl et al. (2012b) reported greater environmental benets interms of CC than Plant B due to higher biogas yield (Pschl et al.,2010).In terms of TA, acidifying burdens were very similar regardingmaize silage mono-digestion. Nevertheless, larger differences werefound in manure mono-digestion. However, credits due to avoidedconventional management and burdens owing to digestate man-agement were the major contributors in De Vries et al. (2012b) aswell as in Plant A.The results obtained in these studies are very similar in terms ofFE regardless the feedstock. Regarding ME, the best results wereattained by Plant A. Besides credits related to avoided manureconventional management, Plant A also accounts credits providedby avoided nitrate leaching due to mineral fertilisation. The worstresult was reported by Poeschl et al. (2012b) with regard to maizesilage mono-digestion due to the lack of credits derived fromavoided nitrate leaching from mineral fertilisation.It is important to note that the anaerobic mono-digestion of pigslurry achieved better environmental results compared with maizesilage. However, pig slurry has other limitations such has its lowerenergy performance and its availability must be ensured. On theother hand, although anaerobic mono-digestion of maize silageattained worse environmental results, it is better from a businesspoint of view due to its higher energy yield. Nevertheless, it isimportant to take into consideration costs associated with theagricultural step. Moreover, it must be ensure that human andanimal nourishment is guaranteed.In addition, the present study provides the opportunity toassess the environmental performance of two different digestatemanagement schemes. Whereas Plant A directly applies un-treated digestate to agricultural land, only the solid fraction ofdigestate is applied in the scheme of Plant B since the liquid andsolid fraction of the digestate are previously separated. Theapplication of untreated digestate involves a larger amount ofdigestate to be stored, delivered and applied as well as morederived emissions to the atmosphere compared with the appli-cation of merely the solid fraction. Nevertheless, a larger amountof digestate to be applied also implies more credits due to theavoidance of mineral fertilisation. Otherwise, the application ofonly the solid fraction of digestate entails the consumption ofelectricity from the grid required in the separation process aswell as less credits related to avoided mineral fertilisation. Takenall of these under consideration, Plant A attained better envi-ronmental results in almost all impact categories due to creditsprovided by avoided mineral fertilisation. However, the separa-tion of digestate into liquid and solid fraction showed up to be amore suitable digestate management scheme regarding preven-tion of acidifying and eutrophicating emissions than the directL. Lij et al. / Journal of Clea10application of raw digestate.Please cite this article in press as: Lij, L., et al., Assuring the sustainable prProduction (2014), includes a traditional biogas dehumidication. In thisalternative scenario, it was proposed to replace the traditionalbiogas dehumidication unit by a chiller based on absorptionprocess. As a result, the electricity consumed inside the plant islower than in the BC (around 15% in both plants).Table 9 displays the comparative results obtained in the pro-posed alternative scenarios for each plant under study.According to the results, surplus heat valorisation by means ofan ORC turbine (AS1) improved the environmental prole of bothbioenergy systems under study compared to BC. These positiveresults are mainly related with the increase in credits provided bythe avoided electricity production (AEP). As expected, the benetsachieved were larger in Plant B than in Plant A due to the increasedelectricity production in Plant B.Table 9Comparative results corresponding with the alternative scenarios. (a) Plant A; (b)Plant B. Acronyms: BC e base case; AS1 e alternative scenario 1; AS2 e alternativescenario 2; AS3 e alternative scenario 3.Category Unit BC AS1 AS2 AS3(a)CC kg CO2 eq 73.5 73.9 72.9 73.8OD kg CFC-11 eq 3.1 106 3.1 106 3.0 106 3.1 106TA kg SO2 eq 0.94 0.94 0.94 0.94FE kg P eq 0.012 0.012 0.012 0.012ME kg N eq 0.77 0.77 0.77 0.77POF kg NMVOC 0.055 0.056 0.056 0.055ALO m2 a 0.32 0.32 0.31 0.32FD kg oil eq 8.9 9.0 8.7 9.0Category Unit BC AS1 AS2 AS3(b)CC kg CO2 eq 15.3 18.5 13.2 16.3OD kg CFC-11 eq 2.7 106 2.4 106 2.9 106 2.6 106TA kg SO2 eq 1.04 1.02 1.05 1.04FE kg P eq 0.015 0.015 0.016 0.015ME kg N eq 0.44 0.44 0.44 0.44POF kg NMVOC 0.44 0.43 0.43 0.43ALO m2 a 23.2 23.2 23.2 23.2FD kg oil eq 7.2 6.2 7.8 6.9oduction of biogas from anaerobic mono-digestion, Journal of Cleanerin the afterburner. Thus, the potential environmental impactstural substrates. Eng. Life Sci. 13 (3), 313e320.Ministerio dello Sviluppo Economico, 2012. Ministerial Decree: Incentives for Non-nerduction capacity showed to be limited because of its biogas pro-duction potential. Besides, AD of maize silage proved to have thecapacity of provide a superior energy potential. Nevertheless, maizecultivation turned out to be a key factor in the environmentalprole of this bioenergy system due to diesel requirements andderived combustion emissions associated with the agriculturalactivities as well as eld emissions from fertiliser application.Ongoing research is being conducted to assess the potentialenvironmental improvement related to the combination of thesefeedstocks in an anaerobic co-digestion process in order to obtainan overall view of all available options.AcknowledgementsThis study was carried out within the framework of the Euro-pean projects LIVE WASTE (LIFE 12 ENV/CY/000544) and Manur-eEcoMine (Project number: 603744). In addition, part of this workwas funded by Regione LombardiadFondo per la Promozione diAccordi Istituzionali, project BIOGESTECA 15083/RCC. The authorsthank Regione Lombardia which nanced a Postdoctoral ResearchFellowship (Progetto Dote Ricerca nanced by FSEdRegioneincreased compared with BC.Finally, the use of a chiller instead of the traditional dehumidi-cation unit (AS3) produced environmental benets compared toBC. These results are related with the decreased electricity con-sumption from the Italian grid in SS3.Therefore, better environmental results were achieved in AS1for almost all impact categories studied in both bioenergy systems.This fact proved that surplus heat is a valuable product with po-tential environmental benets. However in both plants, the ASwhich achieved the best environmental results regarding POF wasAS2 due to the reduction of carbon monoxide emissions.5. ConclusionsThis study analysed the opportunities and drawbacks relatedwith the production of bioenergy from biogas from a livestockwaste (pig slurry) and an energy crop (maize silage). For that pur-pose, two full-scale Italian plants were assessed and compared indetail using LCA methodology. Thus, real input and output owswere identied and managed from a cradle-to-gate perspective inorder to attain the environmental proles of both bioenergysystems.According to the results obtained, the production of bioenergyfrom biogas involves environmental benets since potentiallypolluting processes such as fossil-based electricity and mineralfertilisation could be partially replaced. However, environmentalimpacts were also recognized in processes related to these bio-energy systems such as feedstock production and supply, electricityconsumption, CHP emissions or digestate management.Particularly, AD of pig slurry showed up as an opportunity toattain a number of environmental benets. Not only does it providea renewable energy source and a valuable organic fertiliser, but alsoan appropriate waste management. However, its bioenergy pro-With regard to AS2, the reduction of CO emissions from the CHPthrough the implementation of an afterburner slightly decreasedthe environmental impacts in terms of POF in both bioenergysystems (Table 9). Instead, the remaining impacts categories wereonly inuenced by the decrease in electricity production in thebioenergy production subsystem (SS3), which was translated intolower credits due to electricity production (AEP). This decrease wasmotivated by the fact that part of the biogas produced in AD is usedL. Lij et al. / Journal of CleaLombardia). Dr. Jacopo Bacenetti thanks Regione Lombardia whichPlease cite this article in press as: Lij, L., et al., Assuring the sustainable prProduction (2014), Renewable Energy (in Italian).National Environmental Research Institute, 2010. Danish Emission Inventories forStationary Combustion Plants.nanced a Postdoctoral Research Fellowship (Progetto DoteRicerca nanced by European Social Fund e Regione Lombardia).ReferencesAbubaker, J., Risberg, K., Pell, M., 2012. Biogas residues as fertilisers e effects onwheat growth and soil microbial activities. Appl. Energy 99, 126e134.Althaus, H.J., Hischier, R., Jungbluth, N., Osses, M., Primas, A., 2007. Life Cycle In-ventories of Chemicals. Ecoinvent Report No. 8, v2.0 EMPA. Swiss Centre for LifeCycle Inventories, Dbendorf, Switzerland.Appels, L., Lauwers, J., Degrve, J., Helsen, L., Lievens, B., Willems, K., Van Impe, J.,Dewil, R., 2011. Anaerobic digestion in global bio-energy production: potentialand research challenges. Renew. Sustain. Energy Rev. 15, 4295e4301.Bacenetti, J., Fusi, A., Negri, M., Guidetti, R., Fiala, M., 2014. Environmental assess-ment of two different crop systems in terms of biomethane potential produc-tion. Sci. 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Lij et al. / Journal of Cleaner Production xxx (2014) 1e1212Please cite this article in press as: Lij, L., et al., Assuring the sustainable prProduction (2014), of biogas from anaerobic mono-digestion, Journal of CleanerAssuring the sustainable production of biogas from anaerobic mono-digestion1 Introduction2 Materials and methods2.1 Methodology2.2 Goal and scope definition2.3 Functional unit2.4 Description of systems under assessment2.4.1 Subsystem 1: feedstock production2.4.2 Subsystem 2: feedstock supply2.4.3 Subsystem 3: bioenergy production2.4.4 Subsystem 4: digestate management2.4.5 Avoided processes derived from the valorisation of streams2.5 Life cycle inventory3 Results3.1 Comparative analysis3.2 Detailed assessment of Plant A3.3 Detailed assessment of Plant B4 Discussion4.1 Sensitivity analysis5 ConclusionsAcknowledgementsReferences


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