Laura D’Andrea1*, Pasquale Garofalo1, Alessandro Vittorio Vonella1, Michele Rinaldi2, Angelo Domenico Palumbo1
1 Consiglio per la Ricerca in agricoltura e l’analisi dell’economia agraria – Unità di Ricerca per i Sistemi Colturali degli Ambienti caldo-aridi (CRA-SCA),
via Celso Ulpiani, 5 - Bari, Italy
2 Consiglio per la Ricerca in agricoltura e l’analisi dell’economia agraria – Centro di Ricerca per la Cerealicoltura (CRA-CER), S.S. 673 km 25,200 -
Foggia, Italy
*laura.dandrea@entecra.it
Abstract
The increase of energy demand determined the fossil fuel depletion and the greenhouse emissions (GHG) rising. To tackle these issues, many investigations have been focussed on renewable energy, as energy crops for biofuel production. In this research sweet sorghum for the bioethanol production was cultivated under conventional and reduced energy input (tillage and nitrogen, N). The reduced input cropping system guaranteed good crop performances and bioethanol yield and increased the energy balance and efficiency of the bioethanol supply chain, by ensuring a considerable reduction in Greenhouse Gas (GHG) emission when compared to the conventional crop management.
Keywords: Greenhouse gas; energy balance; energy efficiency; energy crops
Parole chiave: Gas serra; bilancio energetico; efficienza energetica; colture energetiche
Introduction
The primary energy needed worldwide is provided by the fossil fuels for a percentage equal to 80% and the combustion of fossil fuels accounts for 73% of carbon dioxide emissions in the world (Nigam and Singh, 2011). Moreover, the greenhouse emissions (GHG) rising have directed the research on alternative, renewable and with low environmental impact energy sources. Thus EU commission, throughout the Directive “20-20-20 targets”, established that GHG emission and the amount of primary energy should be reduced of 20% and 20% of energy consumption coming from renewable resources (EU Renewable Energy Directive, 2009). The suitability of a crop for energy purposes as sweet sorghum (Sorghum
bicolor L. Moench) for bioethanol production can be
assessed by several energy indicators as the ratio or the difference between the energy output and the input for cultivation, transport and conversion. Moreover, the biofuels can significantly contribute to GHG emissions abatement with a reduction of the GHG emissions of gasoline production pathway of about 67% (Liska et al., 2009). Crop management (tillage and N fertilisation) could heavily impact not only on biomass production but also on energy input and GHG emissions. So, the aim of this research was to assess: i) the biomass and energy yield of sweet sorghum under high and low energy input and ii) compare the GHG emissions of the two cropping systems.
Materials and methods
The field experiment was carried out over the 3-year period from 2010-2012 in Foggia, (Southern Italy) at the experimental farm of CRA-SCA. Sweet sorghum (SUCROS hybrid) was sown at the beginning of May and harvested before heading (August). Irrigation was managed according to the water consumed by plants determined with the gravimetric method. Each time the water used by the crop reached 60 mm irrigation was triggered (199 mm as average seasonal volume). Two energy input systems were compared: High (150 kg N ha-1 and conventional tillage) and Low (no N and no- tillage) treatments. Energy input was determined according to the amount and energy coefficient (RED, 2009) of materials used during crop cultivation, the fuel for transportation of materials (chemicals, seed and biomass) (Tab. 1) and energy to convert sugar in bioethanol (18.33 MJ kg-1 bioethanol; Monti and Venturi, 2003).
Indirect cost of durable items was estimated by considering the lifespan and the energy cost for building and maintenance of each item (Nagy, 1999). The energy output was calculated taking into account the energy density (26.8 GJ t-1; Monti and Venturi, 2003) of potential bioethanol according to Wortmann et al. (2010). The differences between energy output from bioethanol yield and input furnished the net energy gain (NEG); the ratio between the net bioethanol energy output and energy input furnished the energy efficiency (EF). The GHG emissions chain were obtained multiplying the amount of materials and energy in input by their emission coefficient as reported by the RED protocol.
Tab.1 – Amount and energy coefficient of materials used in the sweet sorghum cultivation.
Tab.1 – Quantità e coefficiente energetico dei materiali impiegati nella coltivazione del sorgo zuccherino.
Input level Amount High Amount Low Energy coefficient kg ha-1 MJ kg-1 Seed 15 15 50 Chemical 0 5 268.4 N fertilization 150 0 49.0 P fertilization 100 100 15.2 Diesel (F)* 104.6 55.1 43.1 Diesel (T)** 81.5 73.1 43.1 Irrigation 20*105 20*105 0.00127 *
Diesel for crop management measured at farm and material transportation; **
Trasportation of fresh biomass, assuming a payload of 27 tons and 70 km as field-bioethanol plant distance (short supply chain in Apulia Region).
Results and Discussion
Fig. 1a shows the energy input in the field and that one for transportation of fresh biomass for both managements. For
High treatment the highest energy cost was due to diesel
(36.5%) followed by the N fertilizer (36.2%) and irrigation (12.5%). On the other side, in Low treatment, annulling the energy cost of N and soil tillage, the energy input resulted almost halved if compared with High treatment. However, the main crop parameters and potential bioethanol production resulted not so compromised at lowest energy input with comparable values between treatments (Tab. 2).
Tab.2 – Productivity performance, energy balance and energy efficiency of sweet sorghum.
Tab.2 – Prestazioni produttive, bilancio energetico ed efficienza energetica del sorgo zuccherino.
Treat. FBa DBb Ethanol NEGc EFd
t ha-1 t ha-1 t ha-1 GJ ha-1
High 104.72 26.15 4.15 14.84 1.73
Low 93.94 22.19 3.73 22.28 2.79
a
Fresh biomass; bDry biomass; cNet Energy Gain; dEnergy Efficiency
The NEG resulted in favour of Low treatment (+ 5.4 GJ ha-
1
) with EF improved of 61% if compared with High treatment. As shown in Fig. 1b, the emissions of CO2eq due
to the crop management and transport were equal to 2105 and 1040 kg CO2eq ha-1 for High and Low, respectively.
Essentially, the share of GHG emissions followed the fluxes of materials for the bioethanol supply chain (excluding the conversion process) with the highest impact of N and diesel input in High treatment and diesel and electricity for irrigation in the Low one.
Finally, to produce 1 MJ of energy from the bioethanol supply chain in sweet sorghum, the CO2eq emissions
resulted equal to 18.9 and 10.4 g for High and Low treatment, respectively.
Fig.1 – Energy input (a) and CO2eq emissions (b) for the fluxes of materials in sweet sorghum.
Fig.1 – Input energetici (a) ed emissioni di CO2eq (b) per i diversi flussi di materiali in sorgo zuccherino.
Conclusions
From this research, by reducing the energy input for the cultivation of sweet sorghum not only the crop productivity remained stable, when compared to the conventional cropping system, but also the energy performances resulted boosted. Consequently, the sorghum-bioethanol supply chain in reduced input systems heavily lowered the GHG emission. The low input is a crucial aspect when the substitution of energy from fossil resources by renewable resources has to be evaluated.
Aknowledgements
This work has been supported by Italian Ministry of Agriculture, Food and Forestry Policies (MiPAAF) under BIOSEA project (Ottimizzazione delle filiere bio-energetiche esistenti per una sostenibilità economica e ambientale) (D.M. 16916/7303/10, 23 July 2010).
References
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Monti A., Venturi G., 2003. Comparison of the energy performance of fibre sorghum, sweet sorghum and wheat monocultures in northern Italy. Eur J Agron, 19 (1): 35-43.
Nigam P., Singh A., 2011. Production of liquid biofuels from renewable resources. Progress in Energy and Combustion Science, 37: 52-68. RED; 2009/28/EC; http://eur-lex.europa.eu/legal-
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0 4000 8000 12000 16000 20000 24000 Ene rgy i nput ( M J ha -1 ) Irrigation Seed P N Chemical Machinery Fuel_field Fuel_transp 0 400 800 1200 1600 2000 2400 HIGH LOW Treatment CO 2eq e m is s ion ( k g ha -1 ) Irrigation Seed P N Chemical Fuel_field Fuel_transp Ethanol convers. a b