Assuring the sustainable production of biogas from anaerobic mono-digestion

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<ul><li><p>bio</p><p>ti b</p><p>e Co</p><p>a r t i c l e i n f o</p><p>Article history:Received 14 November 2013Received in revised form5 March 2014</p><p>2008). 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).</p><p>energy generationtralized source offuel which can bef a wide range ofagriculture, live-</p><p>l., 2012; Bacenettiaste available inre (Holm-Nielsenental and socio-</p><p>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</p><p>* Corresponding author. Tel.: 34 881816739.E-mail addresses:,</p><p>Contents lists availab</p><p>Journal of Clean</p><p>.e ls</p><p>Journal of Cleaner Production xxx (2014) 1e12(S. Gonzlez-Garca).1. Introduction</p><p>Climate 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,</p><p>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 xxx</p><p>Keywords:Biogas productionDigestate managementEnvironmental proleMaize silagePig slurry 2014 Elsevier Ltd. All rights reserved.</p><p>Please cite this article in press as: Lij, L., et aProduction (2014), b s t r a c t</p><p>This 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-</p><p>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 be</p><p>improved through surplus heat use as well as technological improvements such as the replacement ofthe traditional dehumidication unit by a chiller.</p><p> 2014 Elsevier Ltd. All rights reserved.Department of Agricultural and Environmenta</p><p>15782 Santiago de Compostela, Spainb l Sciences e Production, Landscape, Agroenergy, University of Milan, Milan, ItalyAssuring the sustainable production ofmono-digestion</p><p>Luca Lij a, Sara Gonzlez-Garca a,*, Jacopo BacenetMara Teresa Moreira a</p><p>aDepartment of Chemical Engineering, Institute of Technology, University of Santiago d</p><p>journal homepage: wwwl., Assuring the sustainable pr/j.jclepro.2014.03.022gas from anaerobic</p><p>, Marco Fiala b, Gumersindo Feijoo a,</p><p>mpostela, Rua Lope Gomez de Marzoa s/n,</p><p>le at ScienceDirect</p><p>er Production</p><p>evier .com/locate/ jc leprooduction of biogas from anaerobic mono-digestion, Journal of Cleaner</p></li><li><p>LCI life cycle inventory</p><p>nermaize, wheat, triticale and sugar beet are being largely cultivatedfor energy purposes.</p><p>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).</p><p>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).</p><p>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 systems</p><p>FU functional unitSS1 subsystem 1SS2 subsystem 2SS3 subsystem 3SS4 subsystem 4CHP combined heat and powerCSTR continuous stirred tank reactorVS volatile solidsCC climate changeOD ozone depletionAbbreviations</p><p>AD anaerobic digestion</p><p>L. 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).</p><p>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.</p><p>2. Materials and methods</p><p>2.1. Methodology</p><p>LCA is a methodological framework useful to determine theenvironmental impacts of a system, product or activity (ISO 14040,</p><p>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.</p><p>2.2. Goal and scope denition</p><p>As 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).</p><p>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).</p><p>The life cycle environmental impacts of both plants were</p><p>TA 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 3</p><p>Production 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.</p><p>2.3. Functional unit</p><p>According 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.</p><p>2.4. Description of systems under assessment</p><p>Fig. 1 outlines the main processes considered within each bio-energy system. All processes involved in both bioenergy systems</p><p>oduction of biogas from anaerobic mono-digestion, Journal of Cleaner</p></li><li><p>were 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.</p><p>2.4.1. Subsystem 1: feedstock productionPig slurry is the mainwaste stream of pig breeding farms and its</p><p>production is unaffected by its valorisation in anaerobic digestion;therefore, its production was excluded from the system boundariesof Plant A (Fig. 1a).</p><p>Concerning Plant B (Fig. 1b), maize is extensively cultivated forenergy purposes in this region provided that its demand for food/</p><p>Table 1Technical data of plants under study.</p><p>Plant A Plant B</p><p>AD District Lodi PaviaFeedstock Pig slurry Maize silageHydraulic retention time (days) 30e40 25e35Organic loading rate (kg VS m3 day1) 0.74 3.34</p><p>CHP Engine power (kWe) 250 520Electrical efciency (%) 35.7 37.0Thermal efciency (%) 51.0 47.1</p><p>Fig. 1. Flowchart and system boundaries of the bioenergy systems under study. (a) Plant A;indicate replaced processes.</p><p>L. Lij et al. / Journal of Cleaner Production xxx (2014) 1e12 3</p><p>Please 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 boxes</p><p>oduction of biogas from anaerobic mono-digestion, Journal of Cleaner</p></li><li><p>feed is completely satised. Accordingly, the assessment of itsproduction included all agricultural activities from eld prepara-tion operations to chopping step.</p><p>2.4.2. Subsystem 2: feedstock supplyThe supply of pig slurry comprised both the collection of pig</p><p>slurry 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 lorry</p><p>as well as biogas losses (up to 1.5%) caused by leakages in valves andpipe connections.</p><p>2.4.4. Subsystem 4: digestate managementThis subsystem comprises the storage of digestate and its</p><p>application 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 into</p><p>L. Lij et al. / Journal of Cleaner Production xxx (2014) 1e124and it is subsequently ensiled and stored. During storage, 2% of themaize is lost.</p><p>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.</p><p>2.4.3. Subsystem 3: bioenergy productionThis subsystem encompassed all inputs and outputs required for</p><p>the 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.</p><p>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.</p><p>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 the...</p></li></ul>


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