Assessment of the influence of energy density and feedstock transport distance on the environmental performance of methane from maize silages

Bioresource Technology 193 (2015) 256–265

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Assessment of the influence of energy density and feedstock transport distance on the environmental performance of methane from maize silages Jacopo Bacenetti a,⇑, Daniela Lovarelli a, Carlo Ingrao b, Caterina Tricase b, Marco Negri a, Marco Fiala a a b

Department of Agricultural and Environmental Sciences – Production, Landscape, Agroenergy, Università degli Studi di Milano, via Giovanni Celoria 2, 20133 Milano, Italy Department of Economics, University of Foggia, Largo Papa Giovanni Paolo II, 1, 71121 Foggia, Italy

h i g h l i g h t s  The environmental impact of three different maize silages for biogas was evaluated.  Increasing extra-farm transport distance and two transport system were considered.  For small distances (<5 km), the whole plant silage shows the lowest impact.  When distances increase, silages with high energy density become the best solution.

a r t i c l e

i n f o

Article history: Received 13 May 2015 Received in revised form 12 June 2015 Accepted 14 June 2015 Available online 18 June 2015 Keywords: Biogas Life Cycle Assessment Anaerobic digestion Biomass supply Renewable energy

a b s t r a c t In Europe, thanks to public subsidy, the production of electricity from anaerobic digestion (AD) of agricultural feedstock has considerably grown and several AD plants were built. When AD plants are concentrated in specific areas (e.g., Northern Italy), increases of feedstock’ prices and transport distances can be observed. In this context, as regards low-energy density feedstock, the present research was designed to estimate the influence of the related long-distance transport on the environmental performances of the biogas-to-electricity process. For this purpose the following transport systems were considered: farm trailers and trucks. For small distances (<5 km), the whole plant silage shows the lowest impact; however, when distances increase, silages with higher energy density (even though characterised by lower methane production per hectare) become more environmentally sustainable. The transport by trucks achieves better environmental performances especially for distances greater than 25 km. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Fossil fuel consumption reduction and Greenhouse Gas (GHG) emission mitigation of are both key issues in the achievement of environmentally sustainable productive systems. In this context, the renewable energy generation can help meet both these ambitious targets. In Europe, the energy policies is increasingly promoting the generation of energy from renewable sources (i.e. EU target of 27% renewable energy by 2030 and 40% of GHG emission reduction) (European Commission, 2014): biogas produced from agricultural biomasses through anaerobic digestion is one of them. Indeed, it has proved to be an interesting solution for energy ⇑ Corresponding author. E-mail address: [email protected] (J. Bacenetti). http://dx.doi.org/10.1016/j.biortech.2015.06.067 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.

generation in rural areas when used locally (Lijó et al., 2014a,b, 2015; Ingrao et al., 2015a; Mauky et al., 2015; Morero et al., 2015). Moreover, the anaerobic digestion (AD) allows an efficient management solution for several waste products, such as organic municipal solid waste, agricultural (e.g., animal slurry) and agro-food residues (e.g., marc, olive pomace, by-products from tomato processing, etc.). As an example, Bacenetti et al. (2015b) documented an environmental impact reduction ranging from 3% to 14% for tomato purée when tomato seeds and skins are used to feed an AD plant whose cogenerated thermal energy is valorised during tomato processing. Lijó et al. (2015) and Battini et al. (2014) evaluated the AD as effective mitigation strategies in dairy farms. Over the years, thanks to public subsidy frameworks, the production of renewable energy from AD of agricultural feedstock has considerably grown and several AD plants have been built. At the end of 2014 in Germany more than 8000 AD plants were

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running (German Biogas Association, 2015), in Italy about 1250 (Negri et al., 2014a), in the United Kingdom 123 (Whiting and Azapagic, 2014). A considerable development of this agro-energy took place also in Sweden, Switzerland and Austria. Most of plants operate in co-digestion and, consequently, are fed with energy crops (mainly cereal silages), agricultural by-products (animal sewages) and residues from the agro-industry. For instance, in the supply chain investigated by Ingrao et al. (2015a) agro-biogas is produced from a feedstock mixture made of animal effluents, maize and triticale silages and tritello, as one of the by-products from durum wheat grain milling. As a matter of fact, the main feedstock for these AD plants are cereal silages, among which maize one is by far the most used in Germany and Italy. Maize silage has high biomass yield, high methane production and ease of storage (Negri et al., 2014a). In Germany, about 650.000 hectares were specifically grown for biogas production (Dressler et al., 2012). In Italy, 10% of the maize area is dedicated to biogas purposes (approximately 10.000 km2 in the 2013 growing season) (Negri et al., 2014a). Nowadays, in Italy, although the public subsidy framework has been reduced, cereal silages, maize in particular, remain the key feedstock for agricultural AD plants. Over the years, the spread of biogas plants, often concentrated in specific areas (such as the provinces of Cremona, Lodi and Mantua), resulted in a growth of concerns about the increasing amounts of agricultural lands that are tilled to feed digesters. Moreover, several AD plants are concentrated in specific areas, thus resulting in an increase of biomasses’ prices and transport distances. For instance, in the case of AD plants located on farms with little agricultural area, which is not sufficient to guarantee the biomass supply of digesters, the only achievable solution is to purchase the feedstock from the market. For the abovementioned reasons, the feedstock is often bought on the market and transported over long distances (Bacenetti et al., 2015c; Lijó et al., 2015). In these cases, even if the economic profitability can be reached thanks to public subsidies, the environmental sustainability of such a solution cannot be taken for granted. Long-distance transports of big amounts of biomass with high water content (and, consequently, low energy density) can deeply affect the environmental performances of the biogas-to-electricity process (Dressler et al., 2012; Bacenetti et al., 2013, 2015c; Lijó et al., 2014a). When the feedstock is transported over long distances, a proper solution could be to buy biomasses with high energy density. The transport of such biomasses (e.g., silages obtained by a high cut or by the ear only) to the AD plants can be more environmentally sustainable than the transport of silage harvested from the whole plant, despite the reduction of biogas producible per unit of area. As regards maize, silage with a higher energy density than the conventional one (produced by harvesting the whole plant) can be produced by harvesting the plant at high cut or only the ear. When the plant is cut at high height (e.g., at 0.75 m), the basal portion, lignified and characterised by low biogas production, is left on the soil, while only the easily fermentable biomass is collected. When only the ear is harvested, silage with high energy density is produced, since only the plant portion rich in starch and organic matter (easily fermentable) and with very high biogas production is harvested. In the last decade, in order to evaluate the environmental performances of agricultural processes, Life Cycle Assessment (LCA) has become more and more employed. LCA is a methodology that aims to analyse products, processes or services from an environmental perspective [ISO 14040, 2006] (Guinée, 2002; ISO, 2006), providing a useful and valuable tool for agricultural systems evaluation as well as for biogas process (Patterson et al., 2011; Morero et al., 2015; Hahn et al., 2015). LCA has evolved significantly during the past three decades, so becoming more systematic and robust for both identification and

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quantification of the environmental impacts associated with products’ life-cycles (Ingrao et al., 2015b). In this context, the aim of this paper is to analyse the environmental performances of methane production from three different maize silages characterised by different energy density, considering increasing transport distances. Besides, to highlight the environmental hotspots of maize silage production, the main purpose of the study is to evaluate the distance beyond which the transport of silages with higher energy density becomes more environmentally sustainable in respect to the transport of the traditional whole-plant-harvested silage, which allows, however, the highest methane production per hectare. 2. Methods 2.1. Goal and scope definition The goal of this study is to assess the environmental impact of three maize silages characterised by different energy density considering transport distances. The selected cereal crop is the most cultivated in Northern Italy and it is the most used to feed AD plants. The study analyses the following 3 different maize silages that belong to FAO Class 600 and 700: (i) harvest of the whole plant (WP) cut at 10 cm height; (ii) harvest of the plant at a higher cut height (HC) equal to 0.75 m; (iii) harvest of the corn ear only (OE), harvested separately from the rest of the plant. For these three silages, the environmental impacts were evaluated using the LCA methodology considering increasing extra-farm transport distances. With regard to biomass transport, two different solutions were analysed: (i) transport with farm trailers coupled with tractors; (ii) transport with trucks. In more details, the research questions can be summarised as follows: (1) What is the environmental impact, expressed in term of methane production by anaerobic digestion, of maize silage from different plant portions? (2) What are the main hotspots associated with the production of the three maize silages analysed? (3) Beyond which extra-farm distances become more environmental sustainable to transport silage with higher energy density although they show a lower methane production per hectare? The outcomes of such an analysis can be helpful for farmers, farmer associations as well as technicians and local politicians involved in the biogas-bioenergy process. In fact, the achieved results can be useful both for all the agricultural processes in which maize silage is a feedstock (e.g., livestock activities, biogas production by anaerobic digestion) as well as for future studies in which feedstock must be transported. 2.2. Functional unit The functional unit is an important step of any Life Cycle Assessment since it provides the reference to which all other data in the assessment are normalised. With LCA’s application to agricultural processes, different functional units (FUs) can be selected. In many LCA studies of agricultural production systems, the FU is the area (e.g., 1 ha). Nevertheless, the mass-based functional unit is prevalent in LCA studies of agricultural systems. However, the

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mass of silage will not be a proper FU because the three maize silages have different methane production. Therefore, in this study, 1 m3 (20 °C and 1 bar) of potential methane production has been considered as FU. 2.3. System boundaries and cropping system description (v) Maize cultivation was carried out on a farm sited in Lombardy, a region located in the middle of the Po Valley. The global agricultural area of the experimental farm is 62.1 ha and it is cultivated with maize hybrids as single crop (32.3 ha), maize hybrids and winter cereals as double crops (25.5 ha) and ryegrass (4.3 ha). The soil is of medium texture. The average rainfall is 745 mm per year, with two minima in summer and winter. A good water availability is usual in summer as well, as the area is surrounded by mountains and characterised by efficient irrigation systems such as lakes, rivers, canals and ditches (Bacenetti et al., 2015a). As regards cereals cultivation in the Po Valley, the crop cycle is characterised by a standardised sequence of field operations, similar to cultivation practices already analysed in other studies (González-García et al., 2012; Bacenetti et al., 2014; Negri et al., 2014a; Bacenetti and Fusi, 2015; Noya et al., 2015). For each of the maize hybrids analysed, the life cycle has been included within the system boundaries (cradle-to-farm perspective). Therefore, raw materials extraction (e.g., fossil fuels and minerals), manufacture (e.g., seeds, fertilisers, and agricultural machines), use (diesel fuel consumption and derived combustion and tyre abrasion emissions), maintenance and final disposal of machines, and supply of inputs to the farm (e.g., fertilisers and herbicides) were considered. Given the high level of mechanisation of the farm, the indirect environmental burdens of capital goods were also included. The cultivation practices are similar for all the three cases (WP, HC, OE) in study and are summarised as follows: (i) organic fertilisation with 85 t ha 1 of digestate (average composition N = 0.40% P2O5 = 0.08% K2O = 0.31%) carried out with a slurry spreader coupled with a 90 kW tractor (working time 1.9 h ha 1; diesel consumption 44.6 kg ha 1); (ii) soil tillage, a 35 cm depth ploughing (190 kW tractor, 1.3 h ha 1; diesel consumption 22.8 kg ha 1) and two interventions with rotary harrow (130 kW tractor, 0.9 h ha 1; diesel consumption 23.7 kg ha 1); (iii) sowing, carried out with a precision drill seeder with the density of 60,000 plants ha 1 (20 kg ha 1 of seed) (90 kW tractor, 0.7 h ha 1; diesel consumption 5.7 kg ha 1); (iv) crop management, which consists of (i) top mineral fertilisation with prilled urea (60 kg ha 1 – 120 kW tractor, 0.5 h ha 1; diesel consumption 2.1 kg ha 1), (ii) chemical weed control with two treatments of Terbuthylazine

(vi)

(vii)

(viii)

(4 kg ha 1) and Alaclor (1 kg ha 1) (80 kW tractor, 0.6 h ha 1; diesel consumption 3.1 kg ha 1) and (iii) 5 irrigation treatments, the first carried out by means of a travelling sprinkler (600 m3 ha 1) while the remaining ones by surface irrigation (800 m3 ha 1 each) (pump coupled with a 80 kW tractor, 1.2 h ha 1; diesel consumption 10.1 kg ha 1); biomass harvesting at different heights or of the ear only by means of forage harvester, this operation involves the crop cutting and chopping (carried out by means of a self-propelled forage harvester 335 kW, 2.2 h ha 1; diesel consumption 37.5 kg ha 1). For the harvest of the whole plant no modifications of forager settings are needed, whereas for the harvest of high cut silage the cutting system is kept at the specific height (0.75 m). For ear silage, in order to separate the ear from the rest of the plant before the biomass chopping, the forager is equipped with a supplementary cornhusker header; cornstalks chopping with a cornstalk chopper coupled with a tractor (80 kW, 1.5 h ha 1; diesel consumption 9.5 kg ha 1), carried out only where the harvesting was HC and OE. When WP is harvested, no cornstalks chopping is carried out since the harvesting height is too little; transport of the chopped maize, this operation can be carried out directly with farm trailers used also during the harvesting (trailer coupled with tractor goes ahead beside the forage harvester) (90 kW tractor, diesel consumption 6.5 kg ha 1) (Fig. 1 – top) or can be performed with trucks (Fig. 1 – bottom). In this second case, the biomass is transported to the farm with farm trailers (assumed distance 1 km) is unload on the farm square and subsequently loaded up the trucks by means of tractors equipped with frontal loader (90 kW tractor, 0.05 h t 1; diesel consumption 0.11 kg t 1). Biomass losses (2.5% of the harvested biomass) have been considered for this operation, only occurring for transport with truck; biomass ensilage, this operation involves the pressing of the chopped biomass in a horizontal silo made of concrete; it is carried out with 2 tractors equipped with frontal loader (tractors 90 kW, 0.05 h t 1; diesel consumption 0.44 kg t 1). After the pressing, the biomass is covered by plastic film and is stored at least for 2 months before being usable as energy feedstock for the AD plant. During the ensiling, dry matter losses take place.

2.4. Life Cycle Inventory Data concerning field operations, ensilage and transport to the farm were directly measured on farm by means of farmer interviews and farm surveys. In more details all the information about

Fig. 1. Process schematization with particular regard to the two transport systems considered.

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seeds, fertilisers, pesticides and water use were provided by the farmer. Diesel fuel consumption was partly measured (by evaluating the volume of fuel used to fill up fuel tanks to the brim) and partly estimated (for harrowing and top fertilisation) using the model SE3A (Fiala and Bacenetti, 2012) that considers the power requirements of the operative machines and their work capacities. For all the field operations, a proper coupling among tractors and implements was considered. Maize yield of fresh matter for FAO Class 600 and 700 was, for WP 80.54 and 83.98 t ha 1 respectively (29.80 and 30.15 t ha 1 of dry matter, respectively); for HC 45.60 and 60.99 t ha 1, respectively (21.50 and 23.60 t ha 1 of dry matter, respectively); and for OE 22.48 and 20.33 t ha 1, respectively (13.80 and 12.45 t ha 1 of dry matter, respectively). The biomass yields were measured by means of the farms’ weighbridge (Negri et al., 2014b). Dry matter losses during the ensiling greatly depend on ensiling techniques, pressing, type of silos, climatic conditions, etc. With this regards, in accordance with other studies carried out in the same geographic area using the same cereal silage, a dry matter loss equal to 12% has been considered (Borreani and Tabacco, 2014; Bacenetti and Fusi, 2015). Data about chemical characterisation of the different maize silages as well as about their methane potential comes from specific laboratory tests (Negri et al., 2014a). In more details, the Methane Potential (m3 of CH4 per ton of digested silage) was evaluated in Lab-scale unstirred fermenters placed in thermostatic baths at 40 °C (Negri et al., 2014a). The inoculums were collected from different full scale AD plants. The methane potential production for WP, HC and OE are reported in Table 1 for the two maize hybrids. The emissions of N compounds due to fertilisers application were computed according to the model proposed by Brentrup et al. (2000). In more details, emission in air of ammonia and nitrous oxide, and the emission in water of nitrate were evaluated considering soil type, climatic conditions and agricultural management operations. Ammonia volatilisation from organic fertilisers application was assessed considering (i) air temperature, (ii) time between the application and the rainfall or the incorporation in the soil; (iii) infiltration rate according to the fertiliser application circumstances (e.g., presence on crop residues on the soil). NH3 emission from mineral fertiliser application was evaluated taking into account: (i) the type of fertiliser and in more details its ammonium and urea content, (ii) the geographical location of the maize fields and its climatic conditions and soil properties. In more details, as regard to the ammonia volatilisation, considering the digestate dry matter content (5% of fresh matter), spreading on not compacted soil and on cornstalks chopped, a medium infiltration rate was considered (Brentrup et al., 2000). Furthermore, taking into account: (i) the temperature at the digestate spreading time (18 °C), (ii) the timing of soil incorporation (4 h after spreading), (iii) rainfall (no rain occur before soil incorporation); the

Table 1 Methane production from by harvesting the different plant portions. Plant portion

Whole plant (WP) Only ear (OE) Plant cut at 0.75 m (HC) a b

FAO Class

600 700 600 700 600 700

20 °C and 1 bar. Mass fraction of dry matter.

Methanea specific production

Methanea global production

dm3 kg

m3 ha

342.16 351.54 576.43 618.76 442.75 414.58

1b

10,212 10,605 7961 7707 9523 9784

ammonia losses, from application to soil incorporation, are equal to 25% of the applied NH4–N. The emission of nitrous oxide emissions were computed considering the emission factor proposed by the IPCC (IPCC, 2006). Nitrate leaching was evaluated considering (i) nitrogen balance, (ii) field capacity in the effective rooting zone, (iii) rainfall, (iv) drainage water zone. In more details, the nitrogen balance considers the supply of N coming both from the application of mineral and organic fertilisers, as well as from the N released from crop residues mineralisation, and the N removal from the harvested biomass. Therefore, for what concerns HC and OE, where not the whole biomass is harvested, lower N removal has been considered. Phosphate emissions were calculated following Nemecek and Kägi (2007). P emissions vary depending on the land use (e.g., arable land or permanent pastures), on the type of fertiliser applied (e.g., organic or mineral and its specific P composition) and on the type and duration of soil cover (in order to evaluate soil erosion). This model, therefore, calculates the leaching of soluble phosphate to ground water, the run-off of soluble phosphate to surface water and the erosion of soil particles containing phosphorous. Two different phosphorus emissions to water were considered: (i) P leaching to the ground water, assessed using 0.07 kg P ha 1 year 1 as average quantity of P leached to ground water for arable land; (ii) run-off to surface water calculated considering 0.175 kg P ha 1 year 1 as average quantity of P lost through run-off for open arable land. P emissions through erosion to surface waters were not considered due to lack of information about the fraction of the eroded soil that reaches the rivers. Table 2 summarises the data about fertilisation, nutrient removal by the crop and nutrient emissions in air and water. As regard to nutrient removal, it has been accounted considering the following values: 3.75 kg N t1, 4.13 kg N t1 and 8.12 kg N t1 for WP HC and OE respectively (Negri et al., 2014b). Climatic data for the year 2012, necessary for calculating fertilisers emissions, were obtained from the meteorological station closest to the farm. Pesticide derived emissions were also estimated in accordance with Margni et al. (2002). In more detail, the fraction of active substances entering the soil is assumed to be 85% of the total mass applied quantity, around 5% of the pesticide rate stays on the plant and 10% is emitted into the air. The run-off of the active ingredients from the soil into the water is assumed to be 10% maximum of the pesticide rate (Margni et al., 2002). According to Frischknecht et al. (2007) the impact of capital goods was considered when the maintenance and depreciation costs of capital equipment form a substantial part of the product price. Therefore, only the environmental load of capital goods such as tractors, equipments and trucks have been included while the one related to agricultural buildings and infrastructures was excluded. Background data for the production of seed, diesel fuel, fertilisers, pesticides, tractors, equipment and trucks as well as about

Table 2 Fertilisation and related emissions for the three maize cultivations. Parameter

Unit

WP

HC

OE

Chopped maize yield fresh matter Nitrogen content of biomass Digestate application Urea application Nitrate leaching Ammonia volatilisation Dinitrogen oxide Phosphate leaching Phosphate run-off

t ha 1 kg ha t ha 1 kg ha kg ha kg ha kg ha kg ha kg ha

82.23 3.75 85 60 0.00 54.22 3.64 0.346 1.687

53.30 4.13 85 60 86.30 54.22 3.64 0.346 1.687

21.41 8.12 85 60 132.60 54.22 3.64 0.346 1.687

1

1

1 1 1 1 1 1

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transport were obtained from the Ecoinvent Database v.3 (Ecoinvent, 2015). With regard to the amount of tractors and implements, the necessary amounts for each field operation and values such as life span, annual working time and working time for each operation, have been collected at farm and used to modify the Ecoinvent unit process. Empty returns were considered for trailers coupled with tractors as well as for trucks; a sensitivity analysis was done with regard to this aspect (see Section 3.3.1). Considering that the fields were previously dedicated to maize cultivation for silage and grain purpose with similar cultivation practices (e.g., organic fertilisation before sowing and mineral top fertilisation), no change in the overall soil carbon content has been assumed (González-García et al., 2012; Dressler et al., 2012; Bacenetti et al., 2014). 2.5. Life Cycle Impact Assessment (LCIA) The characterisation factors reported by the ILCD method were used. The following nine impact potentials were evaluated according to the selected method: climate change (CC), ozone depletion (OD), particulate matter (PM), photochemical ozone formation (POF), acidification (TA), freshwater eutrophication (FE), terrestrial eutrophication (TE), marine eutrophication (ME) and mineral, fossil and renewable resource depletion (MFRD). 3. Results and discussion 3.1. Field operations and ensiling Table 3 reports the environmental impact for the FU considering the silage produced by the different plant portions (WP, HC and OE) without extra farm transport; the process hotspots are shown in Fig. 2. For all the evaluated impact categories, the methane produced from WP and OE shows the lowest and the highest impacts, respectively. Respect to WP, the FU impacts from HC and OE are 4–8% and 21–33% higher, respectively. The most evident difference is in marine eutrophication, where the environmental impact for HC is 3.3 and the one of OE 5.5 times higher than WP. For all the three case evaluated, the environmental hotspots of the cultivation are fertilisers emissions (mainly for TA, TE, FE, ME and PM), diesel fuel emissions (mainly for CC) and diesel fuel production (mainly for MFRD, OD and POF). The emissions related to fertiliser application (e.g., nitrogen leaching, ammonia volatilisation, phosphorous erosion, etc.) are primarily responsible for TA (with a contribution of 94% for all the three WP, HC and OE), TE (94% for WP, HC and OE), FE (96% for WP, HC and OE), ME (52%, 84% and 89% for WP, HC and OE, respectively). Mechanical field operations play an important role for four of the nine evaluated impact categories, in particular: CC (44% for all the three WP, HC and OE), OD (74%, 78% and 82% on WP, HC and OE, respectively),

POF (86%, 89% and 93% for WP, HC and OE, respectively) and MFRD (84%, 88% and 93% for WP, HC and OE, respectively). With regard to ensiling operation, the impacts are more relevant on OD (13%, 8% and 3% for WP, HC and OE, respectively), POF (11%, 7% and 3% for WP, HC and OE, respectively) and MFRD (14%, 9% and 4% for WP, HC and OE, respectively). Fertiliser production has a low impact on all categories, only affecting more significantly OD (8%, 8% and 9% for WP, HC and OE, respectively). With this regard it should be considered that maize is mainly fertilised with organic fertilisers (whose production has no environmental impact because they are a waste product of another production system: the biogas production). Seed production plays a small role for all the evaluated impact categories (<1.5%) as well as the agro-chemicals production (herbicides and plant protection products) (<1%) except for OD (3.5%, 3.6% and 4.0% for WP, HC and OE, respectively) and for CC (about 1.5% for WP, HC and OE). 3.2. Transport Table 4 reports the environmental impact for the FU (absolute values and variations respect to 0 km of extra-farm transport) when the extra-farm transport is considered; in more detail, a distance of 25 km is considered both for tractors and trucks and one of 100 km only for trucks. All the environmental effects show a linear impact increase at the increase of transport distances; the impact categories mostly influenced by diesel fuel production and consumption (CC, OD, POF and, above all, MFRD) show the highest increases at distance rise, while other impact categories, mainly related to emissions from fertilisers applications (TA, TE, and FE) show a smaller increase. For all the evaluated impact categories the impact increase due to the transport is higher for the FU produced from WP while is smaller for OE. In more details, for both the transport systems, except for ME, this increase for 1 m3 of methane produced from WP is about 1.5 times the one detected for HC and about 3.5–4.0 times the one for OE. Fig. 3 shows the variation in environmental impact for the FU for the impact categories mainly affected by the extra-farm transport distances. 3.2.1. By tractors As regard to the transport with farm trailers coupled with tractors, it is possible to detect the break-even transport distances beyond which the transport of maize silage with a higher energy density (i.e., OE and HC) shows a lower environmental impact respect to silage from the WP. In more details: (1) for CC, until 8 km the environmental impact of 1 m3 of methane produced from WP silage has the lowest impact, while beyond this distance the FU from HC shows the lowest impact; when the distance is longer than 18 km, OE results more interesting than WP as well (Fig. 3– top on the left);

Table 3 Environmental impact for 1 m3 of methane produced from the different plant portions (WP, HC and OE) with an extra-farm transport distance equal to 0 km. Impact category

Unit

WP

Climate change Ozone depletion Particulate matter Photochemical ozone formation Acidification Terrestrial eutrophication Freshwater eutrophication Marine eutrophication Mineral, fossil and ren resource depletion

kg CO2 eq kg CFC-11 eq kg PM2.5 eq kg NMVOC eq molc H + eq molc N eq kg P eq kg N eq kg Sb eq

2.58E 1.83E 4.70E 1.25E 1.86E 8.28E 7.44E 1.01E 3.20E

HC 01 08 04 03 02 02 05 03 06

2.75E 1.93E 5.07E 1.33E 2.00E 8.92E 8.02E 3.33E 3.32E

OE 01 08 04 03 02 02 05 03 06

3.28E 2.22E 6.21E 1.54E 2.46E 1.10E 9.67E 5.55E 3.86E

01 08 04 03 02 01 05 03 06

J. Bacenetti et al. / Bioresource Technology 193 (2015) 256–265

A

B

C

Fig. 2. Hotspot identification for the FU from WP (top), HC (middle) and OE (bottom) (extra-farm transport not included).

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Table 4 Environmental impact increase due to the transport for 1 m3 of CH4 produced from the different maize plant portion (PP) (D = variation respect to 0 km of extra-farm transport). Impact category

PP

Extra-farm distance

Tractor

0 km

Extra-farm distance 25 km

Truck

D %

Extra-farm distance 25 km

D %

Extra-farm distance 100 km

D %

CC kg CO2 eq

WP HC OE

2.58E 01 2.75E 01 3.28E 01

3.96E 01 3.72E 01 3.76E 01

53.5% 35.1% 14.6%

3.09E 01 3.11E 01 3.46E 01

19.8% 13.0% 5.4%

4.42E 01 4.04E 01 3.92E 01

71.3% 46.8% 19.5%

OD kg CFC-11 eq

WP HC OE

1.83E 08 1.93E 08 2.22E 08

3.45E 08 3.06E 08 2.78E 08

88.7% 58.7% 25.3%

2.64E 08 2.49E 08 2.50E 08

44.2% 29.3% 12.6%

4.84E 08 4.03E 08 3.26E 08

164.3% 108.8% 46.9%

PM kg PM2.5 eq

WP HC OE

4.70E 04 5.07E 04 6.21E 04

5.52E 04 5.64E 04 6.49E 04

17.4% 11.3% 4.5%

4.87E 04 5.19E 04 6.26E 04

3.6% 2.3% 0.9%

5.25E 04 5.46E 04 6.40E 04

11.8% 7.6% 3.0%

POF kg NMVOC eq

WP HC OE

1.25E 03 1.33E 03 1.54E 03

2.40E 03 2.13E 03 1.93E 03

91.8% 60.4% 25.4%

1.53E 03 1.53E 03 1.63E 03

22.4% 14.9% 5.9%

2.19E 03 1.99E 03 1.86E 03

74.9% 49.4% 20.7%

AT molc H + eq

WP HC OE

1.86E 02 2.00E 02 2.46E 02

1.96E 02 2.07E 02 2.49E 02

5.3% 3.6% 1.3%

1.88E 02 2.02E 02 2.47E 02

1.1% 0.9% 0.3%

1.94E 02 2.06E 02 2.49E 02

4.1% 2.8% 1.0%

TE molc N eq

WP HC OE

8.28E 02 8.92E 02 1.10E 01

8.68E 02 9.20E 02 1.11E 01

4.8% 3.2% 0.9%

8.37E 02 8.99E 02 1.10E 01

1.1% 0.7% 0.1%

8.58E 02 9.13E 02 1.11E 01

3.6% 2.4% 0.5%

FE kg P eq

WP HC OE

7.44E 05 8.02E 05 9.67E 05

7.81E 05 8.28E 05 9.79E 05

5.0% 3.2% 1.3%

7.49E 05 8.06E 05 9.68E 05

0.7% 0.5% 0.1%

7.60E 05 8.13E 05 9.72E 05

2.1% 1.4% 0.5%

ME kg N eq

WP HC OE

1.01E 03 3.33E 03 5.55E 03

1.37E 03 3.58E 03 5.68E 03

36.0% 7.6% 2.3%

1.03E 03 3.39E 03 5.58E 03

1.9% 1.7% 0.6%

1.03E 03 3.52E 03 5.65E 03

1.9% 5.6% 1.7%

MFRD kg Sb eq

WP HC OE

3.20E 06 3.32E 06 3.86E 06

1.02E 05 8.21E 06 6.28E 06

218.6% 147.3% 62.6%

5.17E 06 4.70E 06 4.54E 06

61.6% 41.6% 17.6%

1.02E 05 8.22E 06 6.28E 06

218.9% 147.5% 62.7%

Fig. 3. Variation of the environmental impact for the impact categories mainly affected by the extra-farm transport for both the considered transport systems (farm trailers coupled with tractors and trucks).

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(2) for OD, methane from HC gets more interesting than from WP at 3 km extra farm transport distance, while OE gets more interesting than WP at 7 km; beyond 12 km OE shows better results than HC as well (Fig. 3 – top on the right); (3) for POF, until 3 km WP is the best solution, from 4 km to 11 km the FU from HC has the lowest impact, while beyond this distance methane from OE shows the lowest environmental burdens. The break-even distance between WP and OE is 7 km (Fig. 3 – bottom on the left); (4) for MFRD the break-even distances occur within few km distance: already at 1 km the methane from HC shows a lower impact than the one from WP, until 6 km HC is the best solution, and beyond this extra farm distance the FU from OE shows the lowest environmental burdens, with the break-even distance between WP and OE equal to 3 km (Fig. 3 – bottom on the right); (5) for PM, TA, TE, FE and ME, no lines cross each other along the evaluated extra farm transport distance; the methane from WP always shows the lowest environmental impact. 3.2.2. By trucks When the extra farm transport is performed with trucks, for all the evaluated impact categories, there is a linear increase. For transport with trucks, respect to that with trailers and tractors, the lines show a smaller slope and the impact increase is lower when the same extra-farm distance is considered. However, also for trucks, the break-even transport distance among the methane produced from the different maize feedstock can be identified, in details: (i) for what concerns CC the environmental impact of 1 m3 of methane transported for distances below 29 km is more interesting for WP; beyond this distance, transporting methane from HC has a lower impact than WP, which also results lower at 57 km for OE instead of WP and beyond 80 km for OE instead of HC (Fig. 3 – top on the left); (ii) for OD, methane from HC has the break-even distance with WP beyond 8 km; OE is better than WP and HC at distances longer than 18 km and 25 km, respectively (Fig. 3 – top on the right); (iii) for POF impact category, beyond 23 km methane from HC becomes more interesting than transporting the one from WP, while beyond 44 km the methane from OE has a lower impact than WP and beyond 61 km also lower than HC (Fig. 3 – bottom on the left); (iv) for MFRD the break-even distance between methane from HC and from WP results already after 2 km; beyond 10 km methane from OE shows lower impacts than WP and beyond 18 km lower than HC as well (Fig. 3 – bottom on the right); (v) for PM, TA, TE, FE and ME, the methane from the three biomasses lines never cross each other within the evaluated extra-farm transport distance; the methane from WP always has the lowest environmental impact. 3.3. Discussion The achieved results about the production of maize silage highlighted the importance of energy density when the chopped maize is transported. The methane from OE, although it involves a lower methane production per hectare, can be the best solution for a low environmental impact when extra-farm transport is performed. When the transport is considered, the specific methane production and, consequently, the energy density of the feedstock, is a key aspect able to offset the differences regarding the methane production per hectare. Respect to the FU, thanks to the different methane production (126.4 m3 t 1 fresh matter, 188.0 m3 t 1 of fresh

matter, 348.7 m3 t 1 of fresh matter) (Negri et al., 2014b), transporting OE and HC has an environmental impact considerably lower in respect to WP (-30% and -65% for HC and OE, respectively). For all the evaluated impact categories, when increasing extra-farm transport distances are considered, the environmental impact of the FU increases. However, this growth is faster for the methane produced from maize feedstock characterised by lower energy density (WP) due to the lower specific methane production. For CC, OD, POF and MFRD, WP achieves the better environmental performances only for short distances but, when the distance increases, the silages with higher energy density (HC and OE) show lower environmental impact. However, for the impact categories affected by N and P emissions such as AP, TE, ME and FE to harvest the whole plant stays the most environmentally friendly solution also for considerable extra-farm distances (>100 km). The comparison between the two transport alternatives shows that, even for small extra-farm distances, the use of trucks achieves better environmental performances respect to the transport by farm trailers coupled tractors; this is due to the considerably higher environmental load associated with the transport by tractors. For distances greater than 2–20 km (depending on the impact category), to transport the chopped maize with trucks achieves better environmental performances respect to the use of farm trailers, this occurs even if 2.5% of the harvested biomass is lost due to unloading and loading operations. Nevertheless, it must be considered that knowing the machines that compose the usual farm machinery fleet of North-Italian farms, it is not realistic to assume that farmers, usually equipped with tractors and trailers, resort trucks for transporting biomass. Nevertheless, this solution can be interesting only for long extra-farm transport distances and when a limited number of trailers and tractors is available in the farm machinery fleet or during the harvesting. 3.3.1. Sensitivity analysis To test the robustness of the achieved results, a sensitivity analysis was carried out. A set of parameters/conditions were changed, and their influence on the environmental impact of the FU was evaluated. In more detail, to run the sensitivity analysis: (i) as regard to the methane volume per hectare, the minimum and maximum values recorded in the experimental tests (see Table 1) were considered; (ii) concerning the extra farm transport, both for trailers coupled with tractors and trucks, the load factor for the return trip (0% in the Baseline Scenario – empty return) was increased to 50% considering the possibility to transport other feedstock/goods; (iii) with regard to the fertilisation, considering that the production of maize silage from OE is characterised by higher nutrient losses (e.g., nitrate leaching) due to a lower nutrient removal by the crop, the possibility to differently fertilise the crop according the harvested plant portion has been evaluated. Respect to the baseline scenario (see Table 2), fertilisation has been reduced to 55 t ha 1 of digestate (keeping constant the urea application) for HC, and to 40 t ha 1 of digestate and 45 kg ha 1 of urea for OE; Table 5 reports the emissions calculated according to these fertilisation schemes. Table 5 Emissions related to fertilisers application. Parameter

Unit

Nitrate leaching Ammonia volatilisation Dinitrogen oxide Phosphate leaching Phosphate run-off

kg ha kg ha kg ha kg ha kg ha

1 1 1 1 1

HC

OE

1.40 36.64 2.46 0.300 1.281

0.10 26.67 1.80 0.276 1.078

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Fig. 4. Results of the sensitivity analysis as regard to the different fertilisations. BS: fertilisation as described in Table 2; AS: fertilisation based on nutrient removal (see Table 5).

As regard to the methane production per hectare, when minimum and maximum values are considered the environmental impact of the FU produced from WP, HC and OE varies consequently, when the minimum is considered the impact increase is equal to 1.92% for WP, 1.37% for HC and 1.65% for OE; while, when the maximum production is taken into account the impact decreases of 1.85%, 1.33% and 1.60% for WP, HC and OE, respectively. Concerning transport, as expected, the increase of the load factor for the return trips reduces the environmental impact related to the extra-farm transport of maize biomass. In more details, this reduction is equal to the 25% of the environmental burdens related to extra-farm transport and moves forward the break-even distances for the three maize feedstock, both for tractors and trucks. In others words, for the FU from WP, the feedstock remains for a high amount of km driven the one with the lowest environmental impact, while, only for longer distances, OE becomes the most sustainable option. As regard to these results, it must be underlined that the possibility to increase the load factor for the return trip is easy when trucks carry out the transport. Instead, when the feedstock is transported to the storage directly from the field by means of trailers coupled with tractors a quick return is needed in order to allow the forage harvester to work without interruptions. This reduces the possibility to transport other feedstock and goods during the return trips. When the extra-farm transport is performed with trucks, the temporarily storage of the biomass at farm, although involves losses of biomass (due to unloading from the tractor trailer and loading on the truck), allows a more elastic management of the transport and, so, the possibility to transport other feedstock/goods. Fig. 4 reports the results of the sensitivity analysis as regard to the different fertilisations. When the fertilisation is carried out considering the nutrient removal by the crop, the environmental impact of FU from HC and OE is strongly reduced mainly for the impact categories deeply affected by nitrate leaching, ammonia volatilisation and phosphorous losses (e.g., AT, FE, TE and ME). This reduction is higher for OE than for HC because, in this latter, the nutrient losses in the baseline scenario are lower. In more details, respect to the baseline scenario, for HC the impact is reduced of 27.7% for PM, 31.2% for TA and TE, 20.7% for FE and 73% for ME; while, for OE, it is 44.0% for PM, 48.6% for TA and TE, 30.8 for FE and 84.1% for ME.

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