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Assessment of GHG emissions of biomethane from energy cereal crops in Umbria, Italy
Applied Energy 108 (2013) 128–136
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Assessment of GHG emissions of biomethane from energy cereal crops in Umbria, Italy C. Buratti ⇑, M. Barbanera, F. Fantozzi CRB – Biomass Research Centre, via G.Duranti – Strada S. Lucia Canetola s.n., 06125 Perugia, Italy
h i g h l i g h t s GHG emissions of biomethane from energy crops cultivated in a central Italian farm were investigated. Electricity consumption of the biogas plant was monitored. Current scenario does not allow to achieve a GHG saving according to Renewable Energy Directive. GHG emissions could be reduced by covering the storage tanks of digestate and installing a CHP plant.
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Article history: Received 5 November 2012 Received in revised form 12 February 2013 Accepted 4 March 2013 Available online 3 April 2013 Keywords: Biomethane Energy crops GHG emissions Biogas RED Directive
a b s t r a c t Biomethane from energy crops is a renewable energy carrier and therefore it potentially contributes to climate change mitigation. However, significant greenhouse gas (GHG) emissions resulting from cultivation and processing must be considered. Among those, the production and use of nitrogen fertilizers, the resulting nitrous oxide (N2O) emissions, the methane emissions from digestate storage and the energy consumption of the biogas plant are crucial factors. In the present paper an integrated life cycle assessment (LCA) of GHG emissions from biomethane production is carried out, taking into account own measurements and experience data from a modern biogas plant located in Umbria, Italy. The study is also focused on the electricity consumption of the biogas plant, assessing the specific absorption power of each machinery. The analysis is based on the methodology defined by the European Union Renewable Energy Directive 2009/28/EC (RED). The main result is that the biomethane chain exceeds the minimum value of GHG saving (35%) mainly due to the open storage of digestate. However by varying the system, using heat and electricity from a biogas CHP plant and covering digestate storage tank, a reduction of 68.9% could be obtained. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The interest in renewable energy sources has strongly increased due to the need to reduce fossil energy consumption and greenhouse gas (GHG) emissions, in compliance with the Directive 2009/28/EC on the promotion of the use of energy from renewable sources in the EU (Renewable Energy Directive, RED) [1]. It establishes an overall binding target of a 20% share of renewable energy sources in energy consumption and a 10% binding minimum target for renewable energy sources in transportation to be achieved by each Member State, as well as binding national targets by 2020, in agreement with the overall EU target of 20%. RED introduces environmental sustainability criteria for biofuels and other bioliquids that should be met to obtain the targets ⇑ Corresponding author. Tel.: +39 0755853993. E-mail addresses: [email protected] (C. Buratti), [email protected] (M. Barbanera), [email protected] (F. Fantozzi). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.03.011
and to benefit from national support systems. In particular the GHG emission saving by using biofuels and other bioliquids must at least 35% for current biofuels, at least 50% with effect from 1 January 2017 and 60% from 1 January 2018 for installations with starting production after 1 January 2017. A methodology to calculate GHG emissions from biofuels and other bioliquids as well as the GHG emissions reduction when compared to the replaced fossil fuels is also introduced. The Italian National Renewable Energy Action Plan (PAN, 2010) [2] defines guidelines to achieve the targets at 2020. According to the PAN, the gross final consumption of energy for Italy in 2008 amounted to 131.6 Mtoe, with a growth forecast for 2020 up to 145.6 Mtoe. Consistent with the provisions of Law 99/2009 [3], a further effort is expected from energy efficiency actions that would bring the gross final consumption in 2020 to a value of 133 Mtoe. Energy consumption in the transport sector, subject to the requirement of 10%, is equal to 33,973 ktoe in 2020 and it will be achieved mainly by means of biofuels, for which a production of 600 ktoe for bioethanol, 1880 ktoe for biodiesel and
C. Buratti et al. / Applied Energy 108 (2013) 128–136
50 ktoe for biomethane and vegetable oils was assumed [2]. Natural gas vehicles are widely used in Italy (about 660,000, equal to 1.8% of the overall fleet), with a consumption of about 700 ktoe [4], therefore a higher biomethane consumption in 2020 could be expected, with respect to the value provided by the PAN. This is also justified by the diffusion of biogas plants in Italy, especially in the agricultural sector, with 521 anaerobic digestion plants (391 operating and 130 under construction) in 2011 [5]. Several studies about Life Cycle Assessment (LCA) and RED application were carried out at the Biomass Research Centre (University of Perugia) in the recent years [6–8]. The aim of the present paper was to evaluate the environmental sustainability of biomethane produced from energy crops, in terms of GHG balance, applying the methodology recommended by the RED, which gives GHG emissions default values only for supply chains from municipal waste and manure. An Italian anaerobic digestion plant, fed only by energy crops (corn, triticale and sorghum), was monitored to collect experimental data, in order to carry out a reliable GHG emissions assessment. The examined plant is equipped with a comprehensive process control system, recording data such as substrate input as well as gas and energy flows, in all stages. The analysis was based on a 1-year operation period. The plant produces biogas for electricity generation by means of an internal combustion engine, therefore Literature data were assumed for the upgrading process, generally performed in order to remove CO2, adjusting the calorific value and relative density, in order to meet the standards for use as a vehicle fuel. Agricultural data used in the GHG balance are referred to the years 2010–2011. N2O emissions from the nitrogen fertilizers application were estimated by the IPCC model (Tier 1) [9]. 2. Materials and methods 2.1. European RED Directive The RED methodology was described in a previous work [8]. Each chain is divided into three steps (cultivation, processing and transport) and the reference GHG emission values (typical and default values – Annex V, part D) are indicated for the global production chain. The typical value is an estimate of the representative GHG emission for a particular biofuel production pathway, while the default value is calculated by multiplying the typical value for processing emissions by 1.4. The GHG taken into account are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), considering the following conversion coefficients to CO2eq: 1 for CO2, 23 for CH4 and 296 for N2O and are expressed as grams of equivalent CO2 per MJ of fuel. The actual GHG emissions values are calculated as follows:
Eb ¼ eec þ el þ ep þ etd þ eu esca eccs eccr eee
ð1Þ
where Eb is the total emissions from the use of the biofuel (g CO2 eq. MJ1); eec the emissions from the cultivation of raw materials (g CO2 eq. MJ1); el the emissions from carbon stock changes caused by land use change normalized to 1 year (g CO2 eq. MJ1); ep the emissions from processing (g CO2 eq. MJ1); etd the emissions from transport and distribution (g CO2 eq. MJ1); eu the emissions from fuel in use (g CO2 eq. MJ1); esca the emission savings from soil carbon accumulation via improved agricultural management (g CO2 eq. MJ1); eccs the emission savings from carbon capture and geological storage (g CO2 eq. MJ1); eccr the emission savings from carbon capture and replacement (g CO2 eq. MJ1); and eee is the emission savings from excess electricity from cogeneration (g CO2 eq. MJ1). Then Eb is used to calculate the GHG saving achievable by replacing fossil fuel comparator with the biofuel:
GHG Saving ¼ ðEF Eb Þ=EF
ð2Þ
129
where EF is the total emissions from the fossil fuel comparator (83.8 g CO2 eq. MJ1). In this work the terms el and esca are considered as zero, because there is no land use change (the cultivation of energy dedicated cereals is associated to replace similar crops) and standard agricultural operations were assumed. Carbon capture, sequestration or replacement are also not considered for the biomethane chain, and therefore the emission savings eccs or eccr are also equal to zero. Furthermore RED (paragraphs 17 and 18 of part C, Annex V) states that the allocation of emissions between the products inside the system boundary shall be carried out in proportion to the energy content of the products (determined by Lower Heating Value – LHV for co-products other than electricity).
2.2. Description of the examined plant Several studies about the overall GHG emissions and LCA of biogas or biomethane based on energy crops are available in the Literature [10–15]. Arnold [16] and Koponen et al. [17] showed that the comparison of the results is rather difficult, as LCA depends strongly on the feedstock used, the technology applied and the assumptions taken for the agricultural and technical aspects. Moreover, the functional unit and the system boundaries are usually different. The relevant assumptions for the presented analysis are laid out in this section. Experimental data were taken from a modern 1 MWe biogas plant located in Bevagna (42.58°N, 12.36°E), in Umbria region (Central Italy) (Fig. 1). The reference flow sheet and the boundaries of the system are shown in Fig. 2. The facility’s digestion system consists of two completely stirred tank reactors and one secondary digester, operating in mesophilic conditions (42 °C), and with an approximate retention time of 75 days for the first phase and 85 days for the second one. Biogas plant is fed only by energy crops (maize (Zea mays), triticale (Triticum Secale) and sorghum (Sorghum vulgare)), cultivated in two different sites: Torgiano (43.01°N, 12.26°E) and Bevagna (near the biogas plant). The first site covers an area of 120 ha and it is cultivated as follows: – 81 ha: First Harvest (FH) maize (from May to August); – 14 ha: Second Harvest (SH) sorghum (from May to August) and triticale (from November to July); – 25 ha: FH sorghum (from May to August). The second site has an area of 140 ha and it is cultivated as follows: – 7 ha: FH maize (from May to August); – 62 ha: FH sorghum (from May to August); – 21 ha: triticale (from November to April) and SH sorghum (from May to October); – 50 ha: triticale (from November to April) and SH maize (from May to October). Biogas produced in the fermentation process is cooled down to reduce the water content by condensation and sent in a 1 MWe gas engine to produce electricity. In this study, however the focus is on GHG reduction from biomethane use in transportation, a different process was considered: biogas is firstly sent to a dehumidification and desulphurization section and then to the upgrading plant for purification via pressurized water scrubbing; finally it is transported to a filling station where it is compressed. It is assumed that the filling station is located either on the same site where the anaerobic digestion plant and upgrading facility or nearby on the same local distribution network. An EU report [18] shows that the energy required for natural gas local distribution is zero, due to the high pressure
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Fig. 1. The analyzed biogas plant.
of pipelines that feed low pressure networks and provide sufficient pressure energy to supply local distribution.
3. Input data 3.1. Crop farming Data related to the energy cereals cultivation were obtained from surveyed farms, during the growing period 2010–2011. Table 1 shows the agricultural field activities for each crop. Fertilization was carried out in different ways at Bevagna and Torgiano: liquid digestate from the biogas plant was used at Bevagna, while solid digestate together with urea (46% N) and diammonium phosphate (18% N, 46% P2O5) were employed at Torgiano. Table 2 shows the characteristics and the composition of the liquid and solid fractions of the digestate (data were taken from the analysis report provided by the farm). Liquid digestate was applied through a hose-reel irrigation machine, coupled with a 15 kW pump. The hose-reel has a length of 400 m and a speed of 70 m h1. A 50 kW tractor was also used for moving the reel and the pump from one pull to the next. The
spreading of 5 mm per hectare of digestate required 2.8 h and the corresponding electricity consumption was 42 kW h ha1. Table 3 shows the fertilization plan adopted at Bevagna and Torgiano. Irrigation was carried out using two methods. The first system was employed on 50 ha at Torgiano and 35 ha at Bevagna; it involves the use of the hose-reel irrigation machine coupled to a 35 kW pump; the same tractor used for the liquid digestate application was employed. The second method was used on the remaining surface and it was constituted by a center pivot system with a 50 kW pump. About 40 mm of water were distributed for each irrigation, with an electricity consumption of 210 kW h ha1 (hose-reel machine) and 112.5 kW h ha1 (center pivot system) respectively. Table 4 shows the irrigation times for each crop during the growing period 2010–2011. Chemical weeding was carried out only for FH maize and FH sorghum, by applying 4 kg ha1 of Lumax and 2 kg ha1 of Primagram TZ respectively. Data on diesel oil consumption in agricultural processes were collected from surveyed farms. Table 5 shows the typical fuel use for each operation declared by the farms. Table 6 provides the average harvestable yield for each crop between Bevagna and Torgiano sites; it was assumed that approximately 10% of the total biomass was not collected and remained on the field. The IPCC Guidelines Tier 1 methodology [9] was used to calculate N2O emissions (direct and indirect). It allows to determine direct and indirect N2O emissions and CO2 from urea distribution. Direct N2O emissions are caused by the use of nitrogen fertilizers and by the degradation of crop residues. The emission factor was assumed equal to 10 g N2O-N kg1 of N applied. In particular, the amount of above and below ground residues and their nitrogen content were calculated with the methodology reported in chapter 11 (Equation (11.6) – Tier 1). For their calculation the following parameters were assumed: – RAG = ratio of above-ground residues dry matter to dry matter yield (0.11 for each crop, since it was assumed that 10% of above-ground biomass was not collected);
Fig. 2. Reference flow of the analyzed system.
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– NAG = N content of above-ground residues dry matter (maize: 0.005 kg kg1 of N [9], sorghum: 0.007 kg kg1 of N [6], triticale: 0.006 kg kg1 of N [12]); – RBG = ratio of below-ground residues dry matter to dry matter yield (maize: 0.22 [9], sorghum: 0.19 [19], triticale: 0.22 [12]); – NBG = N content of below-ground residues dry matter (maize: 0.007 kg kg1 of N [9], sorghum: 0.006 kg kg1 of N [6], triticale: 0.009 kg kg1 of N [12]), Indirect N2O emissions were also taken into account. IPCC Guidelines detail the calculation of the two indirect N2O emissions sources that occur during the agricultural step: N2O due to leaching and runoff (emission factor: 7.5 g N2O-N kg1 of N leached and runoff) and N2O due to NH3 and NOx volatilization (emission factor: 10 g N2O-N kg1 of NH3 and NOx volatilized). The following parameters were considered: – FracGASF = synthetic N fertilizer fraction that volatilizes as NH3 and NOx – the default is 100 g kg1 of N applied [9]; – FracGASM = organic N fertilizer fraction that volatilizes as NH3 and NOx – the default is 200 g kg1 of N applied or deposited [9]; – FracLEACH = N applied fraction lost through leaching and runoff – the default is 300 g kg1 of N applied [14]. The evaluation of the CO2 emissions from urea application was carried out adopting the equation 11.13 of the IPCC Guidelines (emission factor: 200 g kg1 of urea). 3.2. Biogas production Trench silos were used to store crop silages on site. The loading of the trenches is performed with a 104 kW wheel loader during 40 days a year, for about 8 h a day. The same loader is used to load biomass inside the hopper for about 3 h per day. The farm also estimated a diesel oil consumption of about 26.5 L h1. The anaerobic digestion plant is made up of two completely stirred reactors (CSTRs) with insulated walls and a useful volume of 2280 m3 each. The digesters work in parallel and feeds one post-digester with a useful volume of 4275 m3. The mixing inside the three reactors is carried out by two agitators driven by hydraulic motors. Each digester has an internal heating system consisting in steel coiled pipes with circulating hot water from the cogenerator. Digesters are covered by gasometric domes, under which there is plastic netting fitted with air inlet, for biological desulphurisation. The produced digestate is treated on a rotary-sieve-type solid/liquid separator. The liquid fraction is sent to an uncovered underground tank, with a volume of 4800 m3, while the solid one is stored on an impermeable concrete platform. The biogas plant is fed with 52,000 kg of wet biomass per day, consisting of 40% corn, 35% sorghum and 25% triticale. Table 7 shows the main parameters used to calculate the GHG emissions of the crop digestion phase. From the data reported in Table 7 it is possible to obtain an energy yield of 251 GJ ha1 per year. This value is higher than the figure for corn (217 GJ ha1) [20] and reaches the value for the fodder beet (250 GJ ha1) [20]. Energy consumption for the biogas plant management was also evaluated. Thermal energy consumption was observed and an average value provided by the company, equal to 2000 MW h y1, corresponding to about 25% of the available power, as reported in [21,22]. Currently, the plant exploits the heat recovered from the cogeneration engine was assumed, that provides 1 MW of thermal power. The electric consumption of the cogenerator auxiliaries was taken from the plant management system, which provides the installed power of each machinery and the operating time. Table 8
Table 1 Agricultural field operations per year for the different energy crops. Operation
FH maize
SH maize
FH sorghum
SH sorghum
Triticale
Scarification Uprooting Rolling Sowing Pesticide application Fertilization Irrigation Harvesting
X X X X X
– – – X –
X X X X X
– – – X –
X – X X –
X X X
X X X
X X X
X X X
X – X
Table 2 Composition of the solid and liquid digestate used in the field fertilization.
Total solids (%) Total N (% on dry matter basis) P (% on dry matter basis) K (% on dry matter basis)
Solid digestate
Liquid digestate
26.5 1.73 0.72 1.30
2.5 13.60 3.6 0
shows the electricity consumptions, with the exception of those specific to the motor engine. While producing biomethane, however, no engine is present, therefore it was assumed that electricity was drawn from the grid, while thermal energy was supplied by a natural gas boiler, characterized by an overall yield of 92%. In this way, an annual consumption of natural gas of 219,587 Nm3 was obtained. The plant was divided into five sections: silage storage, biomass loading, digesters, digestate management and system management. The total consumption of electricity was estimated at 388.7 MW h y1 during 2011, assuming an operating period of 8000 h. The silage storage section comprises electric pumps for the recovery of silage leachate from the trenches. Such leachate is sent inside the primary digesters and it amounts to about 20 m3 y1. Leachate resulting from the solid digestate storage is recovered and sent to the storage tank of liquid digestate. Potential impact of recovering residual biogas from the open digestate storage was also included. The influence of the digestate storage on GHG emissions of the biomethane chain can be significant, but its determination is not as simple. A research in 61 biogas plants in Germany analyzed the gas production potential of biogas digestates. For this purpose, the effluent of single-stage as well as multi-stage biogas plants was tested in laboratory at 20–22 °C, respectively 37 °C for at least 60 days until gas production decreased significantly. The results ranged from 0.1% (20–22 °C, multi-stage) to 21.8% (37 °C, singlestage) of total CH4 yield [23], while another work by the Institut fur Energie und Umweltforschung (IFEU) states that open storage of digestate emits to the atmosphere 2.5% of methane yield [24]. Nonetheless, the uncertainties in Literature values make difficult to specify the influence of uncovered digestate storage on GHG emissions of the biomethane chain, as reported in [25]. In this paper a study by Liebetrau et al. [26] was considered, in which methane emissions of 10 representative agricultural biogas plants in Germany were monitored, similar to the one analyzed here. Therefore an emission of 4.2% of total methane production (corresponding to 76,197 Nm3 y1) was assumed. Methane leakage from the biogas plant was considered negligible since the plant was built recently (2009). 3.2.1. Biogas purification and upgrading Biogas purification is achieved through a series of processes for the elimination of unwanted elements: mainly H2O, H2S and CO2.
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Table 3 Type and quantity of applied fertilizers. Crop
Bevagna
Torgiano
Type
Quantity
Type
Quantity
FH Maize
Liquid digestate
10 mm (340 kg N ha1)
Urea Diammonium phosphate
SH Maize FH Sorghum
Liquid digestate Liquid digestate
10 mm (340 kg N ha1) 5 mm (170 kg N ha1)
SH Sorghum
Liquid digestate
7.5 mm (255 kg N ha1)
Triticale
Liquid digestate
5 mm (170 kg N ha1)
230 kg N ha1 36 kg N ha1 92 kg P2O5 ha1 76 kg N ha1 – 92 kg N ha1 170 kg N ha1 92 kg N ha1 170 kg N ha1 138 kg N ha1
Table 4 Irrigation plan for Bevagna and Torgiano fields during the growing period in 2010– 2011. Crop
FH Maize SH Maize FH Sorghum SH Sorghum Triticale
Number of irrigation applications Bevagna
Torgiano
4 4 2 2 0
4 – 2 2 0
Solid digestate – Urea Solid digestate Urea Solid digestate Urea
Table 7 Summary of the main inputs for the biogas plant. Component
Quantity
Feedstock Biogas yield Operating time Annual biogas production Biogas LHV Average CH4 content Average CO2 content Average O2 content Average H2S content Average H2 content
17,333 t y1 of wet biomass 179 Nm3 t1 of wet biomass 8000 h y1 3.4 MN m3 19.2 MJ (Nm)3 53.4 vol.% 45.0 vol.% 0.5 vol.% 100 ppm <0.5 vol.%
Table 5 Typical diesel consumption for field operations. Operation
Diesel use (L ha1)
Scarification Uprooting Rolling Sowing Pesticide application Fertilization
52 20 25 12 16 4 (liquid digestate) 5 (solid digestate) 5 (diammonium phosphate) 7 (urea) 5 (for each time) 45
Irrigation Harvesting
Table 6 Crop yield during 2010–2011. Crop
Yield (t ha1)
Moisture (%)
FH Maize SH Maize FH Sorghum SH Sorghum Triticale
54 49 72 72 36
67 67 72 72 74
The removal of H2O and H2S is carried out through state of art technology, already realized in the analyzed plant; therefore the relative power consumptions were taken from the plant monitoring system (Table 9). Water vapor is eliminated through biogas cooling, using a chiller and a heat exchanger. The removal of H2S is carried out by means of biological desulphurization based on the adsorption of hydrogen sulfide in water and the subsequent biological oxidation of the hydrogen sulfide dissolved in water by suspended and immobilized thio-bacteria. Biological desulphurization is integrated into the digesters through the injection of air into their gas space.
Table 8 Summary of the main inputs for the biogas plant. Machinery
Installed power (kW)
Operating time (h y1)
Silage storage 2 Leachate recovery pumps
6
666.7
Biomass loading Belt scraper 2 Conveyor belts Mill hopper 2 Screw conveyors
0.6 5.5 11 5
3200 7066.7 3200 3733.3
44 44 1
2666.7 8266.7 16000
0.77
8000
0.6 0.55
8000 8000
1.5
1333.3
4.8 2.2 0.2 1.0
1333.3 16,000 8000 8000
2.2
4000
3.5 1.5
4000 666.7
Digesters 2 Pumps for biomass transferring 2 Hydraulic pumps 2 Hot water pumps – primary digesters Hot water pump – secondary digester Primary–secondary pump circuit Air pump for biogas desulphurization Condensate recovery pump System management Air compressor 2 programmable controllers PC Air extractors – pump room Digestate management Feed pump of the solid/liquid separator Solid/liquid separator Leachate recovery pump
The removal of carbon dioxide not present in the plant, but necessary to achieve biomethane was considered to be carried out by means of a water scrubbing process, which is currently the most widely used technology in Europe, [27]. Scrubbing CO2 from biogas
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C. Buratti et al. / Applied Energy 108 (2013) 128–136 Table 9 Energy consumption in biogas purification. Machinery
Installed power (kW)
Operating time (h y1)
Air pump for desulphurization Water chiller Biogas blower
0.55
8000
17 7.5
1333.3 8000
Table 10 Emission factors for calculating GHG emissions. Input
Emission factor
Reference
Diesel N fertilizer K2O fertilizer P2O5 fertilizer Pesticides Natural gas Electricity
87.5 g CO2 eq. MJ1 6065.3 g CO2 eq. kg1 583.2 g CO2 eq. kg1 1017.8 g CO2 eq. kg1 17257.6 g CO2 eq. kg1 67.7 g CO2 eq. MJ1 396.4 g CO2 eq. kW h1
[18] [18] [18] [18] [18] [18] [31]
from 53.4% CH4 to biomethane (97% CH4) was assumed to be made by pressurizing the biogas to 10 bar in a water scrubbing tower packed with a high surface area media, in order to provide a high contact area between gas and water. As the raw biogas moves up the column against the flow of water, CO2 dissolves within the liquid stream. Upgraded gas leaves the top of the column. The electricity consumption for water scrubbing was assumed equal to 0.042 kW h kW h1 of biomethane [28] (LHV of biomethane: 33.53 MJ Nm3). Then biomethane is compressed to 200 bar for use as a transport fuel, with an electricity consumption of 0.037 kW h kW h1 of biomethane [29]. Methane losses during upgrading process were also considered, assuming a value of 1% of the purified biogas [28]. A net yearly production of biomethane equal to about 1.85 MN m3 was obtained. 3.3. Biomass and biogas transportation The transportation step comprises two phases: biomass transportation from the field to the biogas plant and biomethane distribution. A truck of 28 Mg with a load capacity of 10 Mg was considered for biomass transportation. The average distance between the fields and the biogas plant was assumed to be 5 km. The travels of empty trucks were also considered. From these assumptions an overall distance of 18,980 km per year was derived, with an overall consumption of 3606.2 kg of diesel oil assuming 0.19 kg km1 [30]. For biomethane distribution, it was assumed that the filling station is located either on the same site as the biogas plant or nearby the local distribution network. 4. Results and discussion
Fig. 3. Percentage of the GHG emissions for the three main stages of the biomethane chain.
For the calculation of the GHG emissions of the biomethane chain, the emission factors shown in Table 10 were assumed [18,31]. Results show that the highest environmental impact is processing, with an environmental burden of 75.1% (Fig. 3 and Table 11). In particular, the most significant contribution is due to the biogas production phase (Fig. 4a), with an impact of 33.05 g CO2 eq. MJ1 (Processes P1 + P2 + P3 + P4 in Table 11), mainly related to open storage of digestate and to thermal energy consumption for digesters heating (respectively 43% and 18.1% of the processing
Table 11 Overall GHG emissions of the biomethane chain (g CO2 eq. MJ1 of biomethane). Phase
Process
Value
%
Cultivation
C1 – Diesel C2 – N fertilizers C3 – P2O5 fertilizers C4 – Pesticides C5 – Electricity C6 – N2O – N fertilizers application C7 – CO2 – urea application Total
2.37 2.35 0.11 0.72 0.13 9.18 0.55 15.41
3.8 3.8 0.2 1.1 0.2 14.6 0.9 24.6
P1 P2 P3 P4
1.74 2.51 8.54 20.26
2.8 4.0 13.6 32.3
P5 – Electricity – biogas purification P6 – Electricity – biogas upgrading P7 – Electricity – biogas compression
0.55 4.62 4.07 4.77 47.06
0.9 7.4 6.5 7.6 75.1
T1 – Biomass transport
0.22 62.69
0.3 100
Processing Biogas production
Biogas treatment
– – – –
Silage storage Electricity – biogas plant Heat – biogas plant Digestate storage
P8 – Biomethane losses Total Transport Total
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It also seems to be very interesting to refer the overall figure of GHG emissions to the three crops and to the site. GHG emissions are related to Nm3 of biogas produced; the latter value was derived assuming the following values of biogas yield for each biomass [33]: – – – –
FH Maize: 649 Nm3 Mg1 of dry matter; SH Maize: 585 Nm3 Mg1 of dry matter; FH and SH Sorghum: 454 Nm3 Mg1 of dry matter; Triticale: 563 Nm3 Mg1 of dry matter.
Bevagna fields show a lower impact than Torgiano’s ones, mainly due to the use of chemical fertilizers. Very interesting is also the comparison between the energy crops, which shows that sorghum has a lower impact despite the lower biogas yield (Fig. 5). The analyzed chain shows a reduction of 25.2% in terms of GHG saving, which does not reach the target value of 35% imposed by RED. It should however be noted that several aspects could be improved as described in the following. 4.1. Storage sealing (Scenario a)
Fig. 4. Percentage of the GHG emissions for the processing (a) and cultivation (b) stages.
As a first improvement, the covering of the digestate storage tanks is considered which is already imposed by the legislation in many Italian regions. Assuming that there is no leakage of biogas from covered tanks, as reported in [32,33], the advantage would be avoiding methane emissions and increasing the final production of biomethane. In this way, GHG emissions would be reduced to 41.28 g CO2 eq. MJ1, with a GHG saving of 50.8%, which would make sustainable the biomethane chain. 4.2. CHP plant – natural gas (Scenario b)
Fig. 5. GHG emissions for the different crops in the two different cultivation sites.
stage). GHG emissions related to the process of purification and upgrading of biogas amounted to 14.01 g CO2 eq. MJ1 (Processes P5 + P6 + P7 + P8 in Table 11), mainly due to the electricity consumption of the water scrubbing plant (4.62 g CO2 eq. MJ1, 9.9%) and to the biomethane compression (4.07 g CO2 eq. MJ1, 8.7%). The biomethane losses from the upgrading facility (P8 in Table 11) lead to an environmental burden not negligible, equal to 4.77 g CO2 eq. MJ1 (10.1% of the Processing stage). GHG emissions of the cultivation step are referred to the biomass mixture (Fig. 4b). It is clear that the greatest weight is constituted by the N2O emissions (59.4% of the cultivation step), resulting from the application of nitrogen fertilizers, while the contribution of the electricity for both fertilization and irrigation is very low (0.8%). Results could be compared with data from the Literature [32], where an LCA analysis of biomethane chain from grass was carried out. A difference in the total GHG emissions of about 10% (69.74 g CO2 eq. MJ1) was found, mainly due to the cultivation phase; in fact different feedstock (grass) was used for the calculations.
The impact on the GHG emissions of a Combined Heat and Power (CHP) plant at the service of biomethane plant was also evaluated. Natural gas CHP plant was assumed (Option b) with electric yield of 35% (ge) and thermal yield of 45% (gt). The term eee in Eq. (1) takes into account emissions savings from excess electricity from a CHP plant. According to RED, in accounting for that excess electricity, the size of the cogeneration unit shall be assumed to be the minimum necessary to supply the heat needed to produce the fuel. Heat demand of the biogas plant was equal to 116.1 kJ MJ1 of biomethane (ET). Electricity (EC) required by the biogas and biomethane plants was 106.9 kJ MJ1 of biomethane. Electricity deliverable by the CHP plant (ECG) was obtained with the following equation:
ECG ¼ ET ðge =gt Þ ¼ 90:3 kJ MJ1 of biomethane
ð3Þ
Therefore, the electricity produced by the CHP plant is less than the one required by the processing step. Natural gas consumption NCHP necessary to feed the CHP plant (ET/gt) is 0.26 MJ MJ1 of biomethane. In this scenario, an amount of electricity, EL = EC ECG equal Table 12 GHG emissions for the assumed scenarios (g CO2 eq. MJ1 of biomethane).
Base case Scenario a: covered digestate storage tanks Scenario b: natural gas CHP plant Scenario c: biogas CHP plant Scenario d: covered digestate storage tanks + biogas CHP plant
GHG emissions (g CO2 eq. MJ1 of biomethane)
GHG saving (%)
62.69 41.28
25.2 50.8
61.99 54.10 26.10
26.0 35.4 68.9
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to 16.6 kJ MJ1 of biomethane, has to be withdrawn from the grid, in order to satisfy the energy consumption of the processing step. GHG emissions of the processing step caused by thermal and electric consumption would drop from 20.29 g CO2 eq. MJ1 of biomethane (sum of P1, P3, P5, P6 and P7 processes in Table 11) to 19.59 g CO2 eq. MJ1 of biomethane; this value (GHGP) was obtained through the following equation:
GHGP ¼ NCHP GHGCH4 þ EL GHGE
ð4Þ
where GHGCH4: natural gas GHG emission factor (67.7 g CO2 eq. MJ1 from Table 10); GHGE: electricity emission factor (396.4 g CO2 eq. kW h1 from Table 10). In this way a benefit of 0.8% is obtained and the GHG saving of the overall chain becomes 26%, however still under the 35% required by the RED.
4.3. CHP plant – biogas (Scenario c) The scenario that would provide the greatest benefit involves the use of a part of the biogas produced to feed the CHP plant (Option c). The heat demand BH could be calculated as follows:
BH ¼ ðH 3; 6=gt Þ=LHVB
ð5Þ
where H is the yearly thermal energy consumption of the biogas plant (2000 MW h y1); and LHVB is the biogas lower heating value (19.2 MJ Nm3). The heat demand could be satisfied with about 0.83 MN m3 of biogas, reducing the yearly production of biomethane from 1.85 MN m3 to about 1.4 MN m3. In this case the electricity required by the processing step EC is 115.9 kJ MJ1 of biomethane and the heat demand of the biogas plant ET amounts to 153.8 kJ MJ1 of biomethane; these specific values, referred to the biomethane unit volume, are higher than the previous because a lower production of biomethane is obtained. Electricity deliverable by the CHP plant ECG, calculated with Eq. (3), is 119.6 kJ MJ1 of biomethane. Therefore, the biogas CHP plant would be able to satisfy both thermal and electric consumption of the processing step, producing an electricity surplus ES = ECG EC, equal to 3.7 kJ MJ1 of biomethane. RED reports that the GHG emission saving associated with that excess electricity shall be taken to be equal to the amount of GHG that would be emitted when an equal amount of electricity is generated in a power plant using the same fuel as the cogeneration unit. The amount of biogas necessary to produce the excess electricity BE = ES/ge and a value of 10.57 kJ MJ1 of biomethane was obtained. The corresponding GHG emissions of the biogas production were GHGB = 45.1 g CO2 eq. MJ1 of biogas, obtained excluding from the overall chain P6, P7 and P8 processes in Table 11 for the part of biomethane used to feed the CHP plant. Therefore the carbon credit BE GHGB obtained was equal to 0.48 g CO2 eq. MJ1 of biomethane; this value has to be subtracted to the overall GHG emissions of the chain. GHG emissions in this scenario are equal to 54.1 g CO2 eq. MJ1 of biomethane, corresponding to a GHG saving of 35.4%, in compliance with the RED requirements until 2017. The best case is represented by the presence of the coverage of the digestate storage tanks and a biogas CHP plant (Option d), which could reduce GHG emissions to 26.1 g CO2 eq. MJ1 of biomethane, corresponding to a GHG saving 68.9% (Table 12). Additional scenarios, not covered by the study, that could improve the sustainability of the biomethane chain are: – changing the digesters configuration reducing retention time of the second reactor and covering the digestate storage tanks, in order to decrease thermal energy consumption for its heating;
Fig. 6. Comparison between the base case and the analyzed scenarios in terms of GHG saving.
– checking the opportunity to increase the percentage of sorghum and triticale of the biomass mixture since they are the energy crops with less environmental impact.
5. Sensitivity analysis Several factors included in this environmental analysis have been identified to significantly affect the result. Some of these factors are based on rather uncertain input data, mainly due to lack of data from monitoring and measurements. In the following section the importance of various critical factors in GHG balance is analyzed further: methane leakage from biogas plant and digestate storage, N2O emissions from nitrogen fertilizers application and biomethane efficiency in existing engines. The process steps where leakage of methane occurs, need to be looked at especially carefully when optimizing the production of biomethane. In the sensitivity analysis the following scenarios were considered: – Scenario 1: biogas leakage from the digester was raised from 0% to 1.5%, according to [16]; – Scenario 2: rather moderate emissions from digestate storage were depicted (4.2%), while here a methane emission up to 15% of total methane production were assumed, according to [25]; – Scenario 3: another parameter of considerable influence on the GWP results is the nitrous oxide emissions from nitrogen fertilizers application. In the above described examinations and results, the emission factors reported in [9] were considered. A lot of recent international research indicates, however, that this data might be much too low. In [34] an emission factor up to 5% of the total N applied is indicated; – Scenario 4: as reported in [35], in existing bi-fuel vehicular engines 1 Nm3 of biomethane displaces 1 l of petrol. However, if biomethane was used in a gas engine its efficiency would improve and 1Nm3 of biomethane could replace 1.22 l of petrol. Therefore GHG saving due to the displacement of fossil fuels would be reduced by about 18%. – Scenario 5: Another aspect to consider is the possibility to choose natural gas as fossil fuel comparator instead of petrol of diesel. A natural gas emission comparator of 67.7 g CO2 eq. MJ1 [18] was assumed. In Fig. 6 GHG results for all scenarios were presented. Results show that the accurate evaluation of methane losses in the various plant sections is crucial for determining the sustainability of the chain.
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6. Conclusions Italy is committed to a number of environmental targets, including 10% market share of renewable energy in transport by 2020. The introduction of biomethane can help to meet these targets. However, its environmental sustainability, as defined by the EU RED, should be verified. The present work focused on the evaluation of the GHG emissions of a biomethane production chain from dedicated energy crops (maize, sorghum and triticale). The main sources of emissions are characterized by the greater degree of uncertainty (N2O emissions from the application of nitrogen fertilizers, heat consumption of the anaerobic digestion plant, CH4 emissions from digestate storage). In the considered scenario, biomethane does not obtain the 35% saving in CO2 emissions required by the RED, mainly due to the digestate storage in uncovered tanks. However, the goal could be easily achieved by using gas-tight closure of the digestate storage tanks (50.8% saving) or employing part of the biogas produced to feed a CHP plant at the service of the biomethane plant (35.4% saving). By combining both solutions a GHG saving of 68.9% is possible to achieve. Therefore results show that the biomethane chain can be an important opportunity to achieve the European targets, especially considering that most of biogas plants are not fed with only dedicated biomass but also with residual biomass (typically manure), which can significantly improve the overall impact. Acknowledgements The authors would like to acknowledge Agricola Iraci Borgia company for assisting with data collection. References [1] Directive 2009/28/EC. On the promotion of the use of energy from renewable sources and amending and subsequently repealing Directives 2001/77/EC and 2003/30/EC.2009.04.23.Off J EU 2009.L 140:16-62. [2] Italian Ministry for Economic Development. Italy National Renewable Energy Action Plan; 2010. [in Italian] [accessed 08.02.12]. [3] Italian government. Law 23 July 2009, n. 99, ‘‘Disposizioni per lo sviluppo e l’internazionalizzazione delle imprese, nonché in materia di energia’’. Published in the Italian Official Gazette n. 176; 31th July 2009 [in Italian]. [4] Consorzio Italiano Biogas. Position Paper – Il Biometano fatto bene: una filiera ad elevata intensità di lavoro italiano; 2012. [in Italian] [accessed 16.05.12]. [5] Fabbri C, Soldano M, Piccinini S. Il biogas accelera la corsa verso gli obiettivi 2020. Inform Agrario Suppl 2011;26:15–9 [in Italian]. [6] Buratti C, Fantozzi F. Life cycle assessment of biomass production: development of a methodology to improve the environmental indicators and testing with fiber sorghum energy crop. Biomass Bioenergy 2010;34:1513–22. [7] Fantozzi F, Buratti C. Life cycle assessment of biomass chains: wood pellet from short rotation coppice using data measured on a real plant. Biomass Bioenergy 2010;34:1796–804. [8] Buratti C, Barbanera M, Fantozzi F. A comparison of the European Renewable Energy Directive default emission values with actual values from operating biodiesel facilities for sunflower, rape and soya oil seeds in Italy. Biomass Bioenergy 2012;47:26–36. [9] Intergovernmental Panel on Climate Change (IPCC). Guidelines for National Greenhouse Gas Inventories, vol. 4. Agriculture, forestry and other landuse. Japan: IGES; 2006.
[10] German Biomass Research Centre. Ökonomische und ökologische Bewertung von Erdgassubstituten aus nachwachsenden Rohstoffen. Leipzig; 2009. [11] Smyth BM, Murphy JD, O’Brien CM. What is the energy balance of grass biomethane in Ireland and other temperate northern European climates? Renew Sustain Energy Rev 2009;13:2349–60. [12] Ahlgren S, Hansson PA, Kimming M, Aronsson P, Lundkvist H. Greenhouse gas emissions from cultivation of agricultural crops for biofuels and production of biogas from manure - Implementation of the Directive of the European Parliament and of the Council on the promotion of the use of energy from renewable sources. SLU Dnr ua 12-4067-08; 2009. [13] Adelt M, Wolf D, Vogel A. LCA of biomethane. J Nat Gas Sci Eng 2011;3:646–50. [14] Thamsiriroj T, Murphy JD. A critical review of the applicability of biodiesel and grass biomethane as biofuels to satisfy both biofuel targets and sustainability criteria. Appl Energy 2011;88:1008–19. [15] Tuomisto HL, Helenius J. Comparison of energy and greenhouse gas balances of biogas with other transport biofuel options based on domestic agricultural biomass in Finland. Agric Food Sci 2008;17:240–51. [16] Arnold K. Greenhouse gas balance of bio-methane – which substrates are suitable? Energy Sci Technol 2011;1(2):67–75. [17] Koponen K, Soimakallio S, Tsupari E, Thun R, Antikainen R. GHG emission performance of various liquid transportation biofuels in Finland in accordance with the EU sustainability criteria. Appl Energy 2013;102:440–8. [18] Edwards R, Larivé JF, Mahieu V, Rouveirolles P. Well-To-Wheels analysis of future automotive fuels and powertrains on the European context. Brussels: Joint Research Centre; 2007. Report No.: version 2c. [19] Zheng X, Liu C, Han S. Description and application of a model for simulating regional nitrogen cycling and calculating nitrogen flux. Adv Atm Sci 2008;25(2):181–201. [20] Murphy JD, Braun R, Weiland P, Wellinger A. Biogas from Crop Digestion, IEA Bioenergy Task 37.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Assessment of GHG emissions of biomethane from energy cereal crops in Umbria, Italy C. Buratti ⇑, M. Barbanera, F. Fantozzi CRB – Biomass Research Centre, via G.Duranti – Strada S. Lucia Canetola s.n., 06125 Perugia, Italy
h i g h l i g h t s GHG emissions of biomethane from energy crops cultivated in a central Italian farm were investigated. Electricity consumption of the biogas plant was monitored. Current scenario does not allow to achieve a GHG saving according to Renewable Energy Directive. GHG emissions could be reduced by covering the storage tanks of digestate and installing a CHP plant.
a r t i c l e
i n f o
Article history: Received 5 November 2012 Received in revised form 12 February 2013 Accepted 4 March 2013 Available online 3 April 2013 Keywords: Biomethane Energy crops GHG emissions Biogas RED Directive
a b s t r a c t Biomethane from energy crops is a renewable energy carrier and therefore it potentially contributes to climate change mitigation. However, significant greenhouse gas (GHG) emissions resulting from cultivation and processing must be considered. Among those, the production and use of nitrogen fertilizers, the resulting nitrous oxide (N2O) emissions, the methane emissions from digestate storage and the energy consumption of the biogas plant are crucial factors. In the present paper an integrated life cycle assessment (LCA) of GHG emissions from biomethane production is carried out, taking into account own measurements and experience data from a modern biogas plant located in Umbria, Italy. The study is also focused on the electricity consumption of the biogas plant, assessing the specific absorption power of each machinery. The analysis is based on the methodology defined by the European Union Renewable Energy Directive 2009/28/EC (RED). The main result is that the biomethane chain exceeds the minimum value of GHG saving (35%) mainly due to the open storage of digestate. However by varying the system, using heat and electricity from a biogas CHP plant and covering digestate storage tank, a reduction of 68.9% could be obtained. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The interest in renewable energy sources has strongly increased due to the need to reduce fossil energy consumption and greenhouse gas (GHG) emissions, in compliance with the Directive 2009/28/EC on the promotion of the use of energy from renewable sources in the EU (Renewable Energy Directive, RED) [1]. It establishes an overall binding target of a 20% share of renewable energy sources in energy consumption and a 10% binding minimum target for renewable energy sources in transportation to be achieved by each Member State, as well as binding national targets by 2020, in agreement with the overall EU target of 20%. RED introduces environmental sustainability criteria for biofuels and other bioliquids that should be met to obtain the targets ⇑ Corresponding author. Tel.: +39 0755853993. E-mail addresses: [email protected] (C. Buratti), [email protected] (M. Barbanera), [email protected] (F. Fantozzi). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.03.011
and to benefit from national support systems. In particular the GHG emission saving by using biofuels and other bioliquids must at least 35% for current biofuels, at least 50% with effect from 1 January 2017 and 60% from 1 January 2018 for installations with starting production after 1 January 2017. A methodology to calculate GHG emissions from biofuels and other bioliquids as well as the GHG emissions reduction when compared to the replaced fossil fuels is also introduced. The Italian National Renewable Energy Action Plan (PAN, 2010) [2] defines guidelines to achieve the targets at 2020. According to the PAN, the gross final consumption of energy for Italy in 2008 amounted to 131.6 Mtoe, with a growth forecast for 2020 up to 145.6 Mtoe. Consistent with the provisions of Law 99/2009 [3], a further effort is expected from energy efficiency actions that would bring the gross final consumption in 2020 to a value of 133 Mtoe. Energy consumption in the transport sector, subject to the requirement of 10%, is equal to 33,973 ktoe in 2020 and it will be achieved mainly by means of biofuels, for which a production of 600 ktoe for bioethanol, 1880 ktoe for biodiesel and
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50 ktoe for biomethane and vegetable oils was assumed [2]. Natural gas vehicles are widely used in Italy (about 660,000, equal to 1.8% of the overall fleet), with a consumption of about 700 ktoe [4], therefore a higher biomethane consumption in 2020 could be expected, with respect to the value provided by the PAN. This is also justified by the diffusion of biogas plants in Italy, especially in the agricultural sector, with 521 anaerobic digestion plants (391 operating and 130 under construction) in 2011 [5]. Several studies about Life Cycle Assessment (LCA) and RED application were carried out at the Biomass Research Centre (University of Perugia) in the recent years [6–8]. The aim of the present paper was to evaluate the environmental sustainability of biomethane produced from energy crops, in terms of GHG balance, applying the methodology recommended by the RED, which gives GHG emissions default values only for supply chains from municipal waste and manure. An Italian anaerobic digestion plant, fed only by energy crops (corn, triticale and sorghum), was monitored to collect experimental data, in order to carry out a reliable GHG emissions assessment. The examined plant is equipped with a comprehensive process control system, recording data such as substrate input as well as gas and energy flows, in all stages. The analysis was based on a 1-year operation period. The plant produces biogas for electricity generation by means of an internal combustion engine, therefore Literature data were assumed for the upgrading process, generally performed in order to remove CO2, adjusting the calorific value and relative density, in order to meet the standards for use as a vehicle fuel. Agricultural data used in the GHG balance are referred to the years 2010–2011. N2O emissions from the nitrogen fertilizers application were estimated by the IPCC model (Tier 1) [9]. 2. Materials and methods 2.1. European RED Directive The RED methodology was described in a previous work [8]. Each chain is divided into three steps (cultivation, processing and transport) and the reference GHG emission values (typical and default values – Annex V, part D) are indicated for the global production chain. The typical value is an estimate of the representative GHG emission for a particular biofuel production pathway, while the default value is calculated by multiplying the typical value for processing emissions by 1.4. The GHG taken into account are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), considering the following conversion coefficients to CO2eq: 1 for CO2, 23 for CH4 and 296 for N2O and are expressed as grams of equivalent CO2 per MJ of fuel. The actual GHG emissions values are calculated as follows:
Eb ¼ eec þ el þ ep þ etd þ eu esca eccs eccr eee
ð1Þ
where Eb is the total emissions from the use of the biofuel (g CO2 eq. MJ1); eec the emissions from the cultivation of raw materials (g CO2 eq. MJ1); el the emissions from carbon stock changes caused by land use change normalized to 1 year (g CO2 eq. MJ1); ep the emissions from processing (g CO2 eq. MJ1); etd the emissions from transport and distribution (g CO2 eq. MJ1); eu the emissions from fuel in use (g CO2 eq. MJ1); esca the emission savings from soil carbon accumulation via improved agricultural management (g CO2 eq. MJ1); eccs the emission savings from carbon capture and geological storage (g CO2 eq. MJ1); eccr the emission savings from carbon capture and replacement (g CO2 eq. MJ1); and eee is the emission savings from excess electricity from cogeneration (g CO2 eq. MJ1). Then Eb is used to calculate the GHG saving achievable by replacing fossil fuel comparator with the biofuel:
GHG Saving ¼ ðEF Eb Þ=EF
ð2Þ
129
where EF is the total emissions from the fossil fuel comparator (83.8 g CO2 eq. MJ1). In this work the terms el and esca are considered as zero, because there is no land use change (the cultivation of energy dedicated cereals is associated to replace similar crops) and standard agricultural operations were assumed. Carbon capture, sequestration or replacement are also not considered for the biomethane chain, and therefore the emission savings eccs or eccr are also equal to zero. Furthermore RED (paragraphs 17 and 18 of part C, Annex V) states that the allocation of emissions between the products inside the system boundary shall be carried out in proportion to the energy content of the products (determined by Lower Heating Value – LHV for co-products other than electricity).
2.2. Description of the examined plant Several studies about the overall GHG emissions and LCA of biogas or biomethane based on energy crops are available in the Literature [10–15]. Arnold [16] and Koponen et al. [17] showed that the comparison of the results is rather difficult, as LCA depends strongly on the feedstock used, the technology applied and the assumptions taken for the agricultural and technical aspects. Moreover, the functional unit and the system boundaries are usually different. The relevant assumptions for the presented analysis are laid out in this section. Experimental data were taken from a modern 1 MWe biogas plant located in Bevagna (42.58°N, 12.36°E), in Umbria region (Central Italy) (Fig. 1). The reference flow sheet and the boundaries of the system are shown in Fig. 2. The facility’s digestion system consists of two completely stirred tank reactors and one secondary digester, operating in mesophilic conditions (42 °C), and with an approximate retention time of 75 days for the first phase and 85 days for the second one. Biogas plant is fed only by energy crops (maize (Zea mays), triticale (Triticum Secale) and sorghum (Sorghum vulgare)), cultivated in two different sites: Torgiano (43.01°N, 12.26°E) and Bevagna (near the biogas plant). The first site covers an area of 120 ha and it is cultivated as follows: – 81 ha: First Harvest (FH) maize (from May to August); – 14 ha: Second Harvest (SH) sorghum (from May to August) and triticale (from November to July); – 25 ha: FH sorghum (from May to August). The second site has an area of 140 ha and it is cultivated as follows: – 7 ha: FH maize (from May to August); – 62 ha: FH sorghum (from May to August); – 21 ha: triticale (from November to April) and SH sorghum (from May to October); – 50 ha: triticale (from November to April) and SH maize (from May to October). Biogas produced in the fermentation process is cooled down to reduce the water content by condensation and sent in a 1 MWe gas engine to produce electricity. In this study, however the focus is on GHG reduction from biomethane use in transportation, a different process was considered: biogas is firstly sent to a dehumidification and desulphurization section and then to the upgrading plant for purification via pressurized water scrubbing; finally it is transported to a filling station where it is compressed. It is assumed that the filling station is located either on the same site where the anaerobic digestion plant and upgrading facility or nearby on the same local distribution network. An EU report [18] shows that the energy required for natural gas local distribution is zero, due to the high pressure
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Fig. 1. The analyzed biogas plant.
of pipelines that feed low pressure networks and provide sufficient pressure energy to supply local distribution.
3. Input data 3.1. Crop farming Data related to the energy cereals cultivation were obtained from surveyed farms, during the growing period 2010–2011. Table 1 shows the agricultural field activities for each crop. Fertilization was carried out in different ways at Bevagna and Torgiano: liquid digestate from the biogas plant was used at Bevagna, while solid digestate together with urea (46% N) and diammonium phosphate (18% N, 46% P2O5) were employed at Torgiano. Table 2 shows the characteristics and the composition of the liquid and solid fractions of the digestate (data were taken from the analysis report provided by the farm). Liquid digestate was applied through a hose-reel irrigation machine, coupled with a 15 kW pump. The hose-reel has a length of 400 m and a speed of 70 m h1. A 50 kW tractor was also used for moving the reel and the pump from one pull to the next. The
spreading of 5 mm per hectare of digestate required 2.8 h and the corresponding electricity consumption was 42 kW h ha1. Table 3 shows the fertilization plan adopted at Bevagna and Torgiano. Irrigation was carried out using two methods. The first system was employed on 50 ha at Torgiano and 35 ha at Bevagna; it involves the use of the hose-reel irrigation machine coupled to a 35 kW pump; the same tractor used for the liquid digestate application was employed. The second method was used on the remaining surface and it was constituted by a center pivot system with a 50 kW pump. About 40 mm of water were distributed for each irrigation, with an electricity consumption of 210 kW h ha1 (hose-reel machine) and 112.5 kW h ha1 (center pivot system) respectively. Table 4 shows the irrigation times for each crop during the growing period 2010–2011. Chemical weeding was carried out only for FH maize and FH sorghum, by applying 4 kg ha1 of Lumax and 2 kg ha1 of Primagram TZ respectively. Data on diesel oil consumption in agricultural processes were collected from surveyed farms. Table 5 shows the typical fuel use for each operation declared by the farms. Table 6 provides the average harvestable yield for each crop between Bevagna and Torgiano sites; it was assumed that approximately 10% of the total biomass was not collected and remained on the field. The IPCC Guidelines Tier 1 methodology [9] was used to calculate N2O emissions (direct and indirect). It allows to determine direct and indirect N2O emissions and CO2 from urea distribution. Direct N2O emissions are caused by the use of nitrogen fertilizers and by the degradation of crop residues. The emission factor was assumed equal to 10 g N2O-N kg1 of N applied. In particular, the amount of above and below ground residues and their nitrogen content were calculated with the methodology reported in chapter 11 (Equation (11.6) – Tier 1). For their calculation the following parameters were assumed: – RAG = ratio of above-ground residues dry matter to dry matter yield (0.11 for each crop, since it was assumed that 10% of above-ground biomass was not collected);
Fig. 2. Reference flow of the analyzed system.
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– NAG = N content of above-ground residues dry matter (maize: 0.005 kg kg1 of N [9], sorghum: 0.007 kg kg1 of N [6], triticale: 0.006 kg kg1 of N [12]); – RBG = ratio of below-ground residues dry matter to dry matter yield (maize: 0.22 [9], sorghum: 0.19 [19], triticale: 0.22 [12]); – NBG = N content of below-ground residues dry matter (maize: 0.007 kg kg1 of N [9], sorghum: 0.006 kg kg1 of N [6], triticale: 0.009 kg kg1 of N [12]), Indirect N2O emissions were also taken into account. IPCC Guidelines detail the calculation of the two indirect N2O emissions sources that occur during the agricultural step: N2O due to leaching and runoff (emission factor: 7.5 g N2O-N kg1 of N leached and runoff) and N2O due to NH3 and NOx volatilization (emission factor: 10 g N2O-N kg1 of NH3 and NOx volatilized). The following parameters were considered: – FracGASF = synthetic N fertilizer fraction that volatilizes as NH3 and NOx – the default is 100 g kg1 of N applied [9]; – FracGASM = organic N fertilizer fraction that volatilizes as NH3 and NOx – the default is 200 g kg1 of N applied or deposited [9]; – FracLEACH = N applied fraction lost through leaching and runoff – the default is 300 g kg1 of N applied [14]. The evaluation of the CO2 emissions from urea application was carried out adopting the equation 11.13 of the IPCC Guidelines (emission factor: 200 g kg1 of urea). 3.2. Biogas production Trench silos were used to store crop silages on site. The loading of the trenches is performed with a 104 kW wheel loader during 40 days a year, for about 8 h a day. The same loader is used to load biomass inside the hopper for about 3 h per day. The farm also estimated a diesel oil consumption of about 26.5 L h1. The anaerobic digestion plant is made up of two completely stirred reactors (CSTRs) with insulated walls and a useful volume of 2280 m3 each. The digesters work in parallel and feeds one post-digester with a useful volume of 4275 m3. The mixing inside the three reactors is carried out by two agitators driven by hydraulic motors. Each digester has an internal heating system consisting in steel coiled pipes with circulating hot water from the cogenerator. Digesters are covered by gasometric domes, under which there is plastic netting fitted with air inlet, for biological desulphurisation. The produced digestate is treated on a rotary-sieve-type solid/liquid separator. The liquid fraction is sent to an uncovered underground tank, with a volume of 4800 m3, while the solid one is stored on an impermeable concrete platform. The biogas plant is fed with 52,000 kg of wet biomass per day, consisting of 40% corn, 35% sorghum and 25% triticale. Table 7 shows the main parameters used to calculate the GHG emissions of the crop digestion phase. From the data reported in Table 7 it is possible to obtain an energy yield of 251 GJ ha1 per year. This value is higher than the figure for corn (217 GJ ha1) [20] and reaches the value for the fodder beet (250 GJ ha1) [20]. Energy consumption for the biogas plant management was also evaluated. Thermal energy consumption was observed and an average value provided by the company, equal to 2000 MW h y1, corresponding to about 25% of the available power, as reported in [21,22]. Currently, the plant exploits the heat recovered from the cogeneration engine was assumed, that provides 1 MW of thermal power. The electric consumption of the cogenerator auxiliaries was taken from the plant management system, which provides the installed power of each machinery and the operating time. Table 8
Table 1 Agricultural field operations per year for the different energy crops. Operation
FH maize
SH maize
FH sorghum
SH sorghum
Triticale
Scarification Uprooting Rolling Sowing Pesticide application Fertilization Irrigation Harvesting
X X X X X
– – – X –
X X X X X
– – – X –
X – X X –
X X X
X X X
X X X
X X X
X – X
Table 2 Composition of the solid and liquid digestate used in the field fertilization.
Total solids (%) Total N (% on dry matter basis) P (% on dry matter basis) K (% on dry matter basis)
Solid digestate
Liquid digestate
26.5 1.73 0.72 1.30
2.5 13.60 3.6 0
shows the electricity consumptions, with the exception of those specific to the motor engine. While producing biomethane, however, no engine is present, therefore it was assumed that electricity was drawn from the grid, while thermal energy was supplied by a natural gas boiler, characterized by an overall yield of 92%. In this way, an annual consumption of natural gas of 219,587 Nm3 was obtained. The plant was divided into five sections: silage storage, biomass loading, digesters, digestate management and system management. The total consumption of electricity was estimated at 388.7 MW h y1 during 2011, assuming an operating period of 8000 h. The silage storage section comprises electric pumps for the recovery of silage leachate from the trenches. Such leachate is sent inside the primary digesters and it amounts to about 20 m3 y1. Leachate resulting from the solid digestate storage is recovered and sent to the storage tank of liquid digestate. Potential impact of recovering residual biogas from the open digestate storage was also included. The influence of the digestate storage on GHG emissions of the biomethane chain can be significant, but its determination is not as simple. A research in 61 biogas plants in Germany analyzed the gas production potential of biogas digestates. For this purpose, the effluent of single-stage as well as multi-stage biogas plants was tested in laboratory at 20–22 °C, respectively 37 °C for at least 60 days until gas production decreased significantly. The results ranged from 0.1% (20–22 °C, multi-stage) to 21.8% (37 °C, singlestage) of total CH4 yield [23], while another work by the Institut fur Energie und Umweltforschung (IFEU) states that open storage of digestate emits to the atmosphere 2.5% of methane yield [24]. Nonetheless, the uncertainties in Literature values make difficult to specify the influence of uncovered digestate storage on GHG emissions of the biomethane chain, as reported in [25]. In this paper a study by Liebetrau et al. [26] was considered, in which methane emissions of 10 representative agricultural biogas plants in Germany were monitored, similar to the one analyzed here. Therefore an emission of 4.2% of total methane production (corresponding to 76,197 Nm3 y1) was assumed. Methane leakage from the biogas plant was considered negligible since the plant was built recently (2009). 3.2.1. Biogas purification and upgrading Biogas purification is achieved through a series of processes for the elimination of unwanted elements: mainly H2O, H2S and CO2.
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Table 3 Type and quantity of applied fertilizers. Crop
Bevagna
Torgiano
Type
Quantity
Type
Quantity
FH Maize
Liquid digestate
10 mm (340 kg N ha1)
Urea Diammonium phosphate
SH Maize FH Sorghum
Liquid digestate Liquid digestate
10 mm (340 kg N ha1) 5 mm (170 kg N ha1)
SH Sorghum
Liquid digestate
7.5 mm (255 kg N ha1)
Triticale
Liquid digestate
5 mm (170 kg N ha1)
230 kg N ha1 36 kg N ha1 92 kg P2O5 ha1 76 kg N ha1 – 92 kg N ha1 170 kg N ha1 92 kg N ha1 170 kg N ha1 138 kg N ha1
Table 4 Irrigation plan for Bevagna and Torgiano fields during the growing period in 2010– 2011. Crop
FH Maize SH Maize FH Sorghum SH Sorghum Triticale
Number of irrigation applications Bevagna
Torgiano
4 4 2 2 0
4 – 2 2 0
Solid digestate – Urea Solid digestate Urea Solid digestate Urea
Table 7 Summary of the main inputs for the biogas plant. Component
Quantity
Feedstock Biogas yield Operating time Annual biogas production Biogas LHV Average CH4 content Average CO2 content Average O2 content Average H2S content Average H2 content
17,333 t y1 of wet biomass 179 Nm3 t1 of wet biomass 8000 h y1 3.4 MN m3 19.2 MJ (Nm)3 53.4 vol.% 45.0 vol.% 0.5 vol.% 100 ppm <0.5 vol.%
Table 5 Typical diesel consumption for field operations. Operation
Diesel use (L ha1)
Scarification Uprooting Rolling Sowing Pesticide application Fertilization
52 20 25 12 16 4 (liquid digestate) 5 (solid digestate) 5 (diammonium phosphate) 7 (urea) 5 (for each time) 45
Irrigation Harvesting
Table 6 Crop yield during 2010–2011. Crop
Yield (t ha1)
Moisture (%)
FH Maize SH Maize FH Sorghum SH Sorghum Triticale
54 49 72 72 36
67 67 72 72 74
The removal of H2O and H2S is carried out through state of art technology, already realized in the analyzed plant; therefore the relative power consumptions were taken from the plant monitoring system (Table 9). Water vapor is eliminated through biogas cooling, using a chiller and a heat exchanger. The removal of H2S is carried out by means of biological desulphurization based on the adsorption of hydrogen sulfide in water and the subsequent biological oxidation of the hydrogen sulfide dissolved in water by suspended and immobilized thio-bacteria. Biological desulphurization is integrated into the digesters through the injection of air into their gas space.
Table 8 Summary of the main inputs for the biogas plant. Machinery
Installed power (kW)
Operating time (h y1)
Silage storage 2 Leachate recovery pumps
6
666.7
Biomass loading Belt scraper 2 Conveyor belts Mill hopper 2 Screw conveyors
0.6 5.5 11 5
3200 7066.7 3200 3733.3
44 44 1
2666.7 8266.7 16000
0.77
8000
0.6 0.55
8000 8000
1.5
1333.3
4.8 2.2 0.2 1.0
1333.3 16,000 8000 8000
2.2
4000
3.5 1.5
4000 666.7
Digesters 2 Pumps for biomass transferring 2 Hydraulic pumps 2 Hot water pumps – primary digesters Hot water pump – secondary digester Primary–secondary pump circuit Air pump for biogas desulphurization Condensate recovery pump System management Air compressor 2 programmable controllers PC Air extractors – pump room Digestate management Feed pump of the solid/liquid separator Solid/liquid separator Leachate recovery pump
The removal of carbon dioxide not present in the plant, but necessary to achieve biomethane was considered to be carried out by means of a water scrubbing process, which is currently the most widely used technology in Europe, [27]. Scrubbing CO2 from biogas
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Installed power (kW)
Operating time (h y1)
Air pump for desulphurization Water chiller Biogas blower
0.55
8000
17 7.5
1333.3 8000
Table 10 Emission factors for calculating GHG emissions. Input
Emission factor
Reference
Diesel N fertilizer K2O fertilizer P2O5 fertilizer Pesticides Natural gas Electricity
87.5 g CO2 eq. MJ1 6065.3 g CO2 eq. kg1 583.2 g CO2 eq. kg1 1017.8 g CO2 eq. kg1 17257.6 g CO2 eq. kg1 67.7 g CO2 eq. MJ1 396.4 g CO2 eq. kW h1
[18] [18] [18] [18] [18] [18] [31]
from 53.4% CH4 to biomethane (97% CH4) was assumed to be made by pressurizing the biogas to 10 bar in a water scrubbing tower packed with a high surface area media, in order to provide a high contact area between gas and water. As the raw biogas moves up the column against the flow of water, CO2 dissolves within the liquid stream. Upgraded gas leaves the top of the column. The electricity consumption for water scrubbing was assumed equal to 0.042 kW h kW h1 of biomethane [28] (LHV of biomethane: 33.53 MJ Nm3). Then biomethane is compressed to 200 bar for use as a transport fuel, with an electricity consumption of 0.037 kW h kW h1 of biomethane [29]. Methane losses during upgrading process were also considered, assuming a value of 1% of the purified biogas [28]. A net yearly production of biomethane equal to about 1.85 MN m3 was obtained. 3.3. Biomass and biogas transportation The transportation step comprises two phases: biomass transportation from the field to the biogas plant and biomethane distribution. A truck of 28 Mg with a load capacity of 10 Mg was considered for biomass transportation. The average distance between the fields and the biogas plant was assumed to be 5 km. The travels of empty trucks were also considered. From these assumptions an overall distance of 18,980 km per year was derived, with an overall consumption of 3606.2 kg of diesel oil assuming 0.19 kg km1 [30]. For biomethane distribution, it was assumed that the filling station is located either on the same site as the biogas plant or nearby the local distribution network. 4. Results and discussion
Fig. 3. Percentage of the GHG emissions for the three main stages of the biomethane chain.
For the calculation of the GHG emissions of the biomethane chain, the emission factors shown in Table 10 were assumed [18,31]. Results show that the highest environmental impact is processing, with an environmental burden of 75.1% (Fig. 3 and Table 11). In particular, the most significant contribution is due to the biogas production phase (Fig. 4a), with an impact of 33.05 g CO2 eq. MJ1 (Processes P1 + P2 + P3 + P4 in Table 11), mainly related to open storage of digestate and to thermal energy consumption for digesters heating (respectively 43% and 18.1% of the processing
Table 11 Overall GHG emissions of the biomethane chain (g CO2 eq. MJ1 of biomethane). Phase
Process
Value
%
Cultivation
C1 – Diesel C2 – N fertilizers C3 – P2O5 fertilizers C4 – Pesticides C5 – Electricity C6 – N2O – N fertilizers application C7 – CO2 – urea application Total
2.37 2.35 0.11 0.72 0.13 9.18 0.55 15.41
3.8 3.8 0.2 1.1 0.2 14.6 0.9 24.6
P1 P2 P3 P4
1.74 2.51 8.54 20.26
2.8 4.0 13.6 32.3
P5 – Electricity – biogas purification P6 – Electricity – biogas upgrading P7 – Electricity – biogas compression
0.55 4.62 4.07 4.77 47.06
0.9 7.4 6.5 7.6 75.1
T1 – Biomass transport
0.22 62.69
0.3 100
Processing Biogas production
Biogas treatment
– – – –
Silage storage Electricity – biogas plant Heat – biogas plant Digestate storage
P8 – Biomethane losses Total Transport Total
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It also seems to be very interesting to refer the overall figure of GHG emissions to the three crops and to the site. GHG emissions are related to Nm3 of biogas produced; the latter value was derived assuming the following values of biogas yield for each biomass [33]: – – – –
FH Maize: 649 Nm3 Mg1 of dry matter; SH Maize: 585 Nm3 Mg1 of dry matter; FH and SH Sorghum: 454 Nm3 Mg1 of dry matter; Triticale: 563 Nm3 Mg1 of dry matter.
Bevagna fields show a lower impact than Torgiano’s ones, mainly due to the use of chemical fertilizers. Very interesting is also the comparison between the energy crops, which shows that sorghum has a lower impact despite the lower biogas yield (Fig. 5). The analyzed chain shows a reduction of 25.2% in terms of GHG saving, which does not reach the target value of 35% imposed by RED. It should however be noted that several aspects could be improved as described in the following. 4.1. Storage sealing (Scenario a)
Fig. 4. Percentage of the GHG emissions for the processing (a) and cultivation (b) stages.
As a first improvement, the covering of the digestate storage tanks is considered which is already imposed by the legislation in many Italian regions. Assuming that there is no leakage of biogas from covered tanks, as reported in [32,33], the advantage would be avoiding methane emissions and increasing the final production of biomethane. In this way, GHG emissions would be reduced to 41.28 g CO2 eq. MJ1, with a GHG saving of 50.8%, which would make sustainable the biomethane chain. 4.2. CHP plant – natural gas (Scenario b)
Fig. 5. GHG emissions for the different crops in the two different cultivation sites.
stage). GHG emissions related to the process of purification and upgrading of biogas amounted to 14.01 g CO2 eq. MJ1 (Processes P5 + P6 + P7 + P8 in Table 11), mainly due to the electricity consumption of the water scrubbing plant (4.62 g CO2 eq. MJ1, 9.9%) and to the biomethane compression (4.07 g CO2 eq. MJ1, 8.7%). The biomethane losses from the upgrading facility (P8 in Table 11) lead to an environmental burden not negligible, equal to 4.77 g CO2 eq. MJ1 (10.1% of the Processing stage). GHG emissions of the cultivation step are referred to the biomass mixture (Fig. 4b). It is clear that the greatest weight is constituted by the N2O emissions (59.4% of the cultivation step), resulting from the application of nitrogen fertilizers, while the contribution of the electricity for both fertilization and irrigation is very low (0.8%). Results could be compared with data from the Literature [32], where an LCA analysis of biomethane chain from grass was carried out. A difference in the total GHG emissions of about 10% (69.74 g CO2 eq. MJ1) was found, mainly due to the cultivation phase; in fact different feedstock (grass) was used for the calculations.
The impact on the GHG emissions of a Combined Heat and Power (CHP) plant at the service of biomethane plant was also evaluated. Natural gas CHP plant was assumed (Option b) with electric yield of 35% (ge) and thermal yield of 45% (gt). The term eee in Eq. (1) takes into account emissions savings from excess electricity from a CHP plant. According to RED, in accounting for that excess electricity, the size of the cogeneration unit shall be assumed to be the minimum necessary to supply the heat needed to produce the fuel. Heat demand of the biogas plant was equal to 116.1 kJ MJ1 of biomethane (ET). Electricity (EC) required by the biogas and biomethane plants was 106.9 kJ MJ1 of biomethane. Electricity deliverable by the CHP plant (ECG) was obtained with the following equation:
ECG ¼ ET ðge =gt Þ ¼ 90:3 kJ MJ1 of biomethane
ð3Þ
Therefore, the electricity produced by the CHP plant is less than the one required by the processing step. Natural gas consumption NCHP necessary to feed the CHP plant (ET/gt) is 0.26 MJ MJ1 of biomethane. In this scenario, an amount of electricity, EL = EC ECG equal Table 12 GHG emissions for the assumed scenarios (g CO2 eq. MJ1 of biomethane).
Base case Scenario a: covered digestate storage tanks Scenario b: natural gas CHP plant Scenario c: biogas CHP plant Scenario d: covered digestate storage tanks + biogas CHP plant
GHG emissions (g CO2 eq. MJ1 of biomethane)
GHG saving (%)
62.69 41.28
25.2 50.8
61.99 54.10 26.10
26.0 35.4 68.9
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to 16.6 kJ MJ1 of biomethane, has to be withdrawn from the grid, in order to satisfy the energy consumption of the processing step. GHG emissions of the processing step caused by thermal and electric consumption would drop from 20.29 g CO2 eq. MJ1 of biomethane (sum of P1, P3, P5, P6 and P7 processes in Table 11) to 19.59 g CO2 eq. MJ1 of biomethane; this value (GHGP) was obtained through the following equation:
GHGP ¼ NCHP GHGCH4 þ EL GHGE
ð4Þ
where GHGCH4: natural gas GHG emission factor (67.7 g CO2 eq. MJ1 from Table 10); GHGE: electricity emission factor (396.4 g CO2 eq. kW h1 from Table 10). In this way a benefit of 0.8% is obtained and the GHG saving of the overall chain becomes 26%, however still under the 35% required by the RED.
4.3. CHP plant – biogas (Scenario c) The scenario that would provide the greatest benefit involves the use of a part of the biogas produced to feed the CHP plant (Option c). The heat demand BH could be calculated as follows:
BH ¼ ðH 3; 6=gt Þ=LHVB
ð5Þ
where H is the yearly thermal energy consumption of the biogas plant (2000 MW h y1); and LHVB is the biogas lower heating value (19.2 MJ Nm3). The heat demand could be satisfied with about 0.83 MN m3 of biogas, reducing the yearly production of biomethane from 1.85 MN m3 to about 1.4 MN m3. In this case the electricity required by the processing step EC is 115.9 kJ MJ1 of biomethane and the heat demand of the biogas plant ET amounts to 153.8 kJ MJ1 of biomethane; these specific values, referred to the biomethane unit volume, are higher than the previous because a lower production of biomethane is obtained. Electricity deliverable by the CHP plant ECG, calculated with Eq. (3), is 119.6 kJ MJ1 of biomethane. Therefore, the biogas CHP plant would be able to satisfy both thermal and electric consumption of the processing step, producing an electricity surplus ES = ECG EC, equal to 3.7 kJ MJ1 of biomethane. RED reports that the GHG emission saving associated with that excess electricity shall be taken to be equal to the amount of GHG that would be emitted when an equal amount of electricity is generated in a power plant using the same fuel as the cogeneration unit. The amount of biogas necessary to produce the excess electricity BE = ES/ge and a value of 10.57 kJ MJ1 of biomethane was obtained. The corresponding GHG emissions of the biogas production were GHGB = 45.1 g CO2 eq. MJ1 of biogas, obtained excluding from the overall chain P6, P7 and P8 processes in Table 11 for the part of biomethane used to feed the CHP plant. Therefore the carbon credit BE GHGB obtained was equal to 0.48 g CO2 eq. MJ1 of biomethane; this value has to be subtracted to the overall GHG emissions of the chain. GHG emissions in this scenario are equal to 54.1 g CO2 eq. MJ1 of biomethane, corresponding to a GHG saving of 35.4%, in compliance with the RED requirements until 2017. The best case is represented by the presence of the coverage of the digestate storage tanks and a biogas CHP plant (Option d), which could reduce GHG emissions to 26.1 g CO2 eq. MJ1 of biomethane, corresponding to a GHG saving 68.9% (Table 12). Additional scenarios, not covered by the study, that could improve the sustainability of the biomethane chain are: – changing the digesters configuration reducing retention time of the second reactor and covering the digestate storage tanks, in order to decrease thermal energy consumption for its heating;
Fig. 6. Comparison between the base case and the analyzed scenarios in terms of GHG saving.
– checking the opportunity to increase the percentage of sorghum and triticale of the biomass mixture since they are the energy crops with less environmental impact.
5. Sensitivity analysis Several factors included in this environmental analysis have been identified to significantly affect the result. Some of these factors are based on rather uncertain input data, mainly due to lack of data from monitoring and measurements. In the following section the importance of various critical factors in GHG balance is analyzed further: methane leakage from biogas plant and digestate storage, N2O emissions from nitrogen fertilizers application and biomethane efficiency in existing engines. The process steps where leakage of methane occurs, need to be looked at especially carefully when optimizing the production of biomethane. In the sensitivity analysis the following scenarios were considered: – Scenario 1: biogas leakage from the digester was raised from 0% to 1.5%, according to [16]; – Scenario 2: rather moderate emissions from digestate storage were depicted (4.2%), while here a methane emission up to 15% of total methane production were assumed, according to [25]; – Scenario 3: another parameter of considerable influence on the GWP results is the nitrous oxide emissions from nitrogen fertilizers application. In the above described examinations and results, the emission factors reported in [9] were considered. A lot of recent international research indicates, however, that this data might be much too low. In [34] an emission factor up to 5% of the total N applied is indicated; – Scenario 4: as reported in [35], in existing bi-fuel vehicular engines 1 Nm3 of biomethane displaces 1 l of petrol. However, if biomethane was used in a gas engine its efficiency would improve and 1Nm3 of biomethane could replace 1.22 l of petrol. Therefore GHG saving due to the displacement of fossil fuels would be reduced by about 18%. – Scenario 5: Another aspect to consider is the possibility to choose natural gas as fossil fuel comparator instead of petrol of diesel. A natural gas emission comparator of 67.7 g CO2 eq. MJ1 [18] was assumed. In Fig. 6 GHG results for all scenarios were presented. Results show that the accurate evaluation of methane losses in the various plant sections is crucial for determining the sustainability of the chain.
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6. Conclusions Italy is committed to a number of environmental targets, including 10% market share of renewable energy in transport by 2020. The introduction of biomethane can help to meet these targets. However, its environmental sustainability, as defined by the EU RED, should be verified. The present work focused on the evaluation of the GHG emissions of a biomethane production chain from dedicated energy crops (maize, sorghum and triticale). The main sources of emissions are characterized by the greater degree of uncertainty (N2O emissions from the application of nitrogen fertilizers, heat consumption of the anaerobic digestion plant, CH4 emissions from digestate storage). In the considered scenario, biomethane does not obtain the 35% saving in CO2 emissions required by the RED, mainly due to the digestate storage in uncovered tanks. However, the goal could be easily achieved by using gas-tight closure of the digestate storage tanks (50.8% saving) or employing part of the biogas produced to feed a CHP plant at the service of the biomethane plant (35.4% saving). By combining both solutions a GHG saving of 68.9% is possible to achieve. Therefore results show that the biomethane chain can be an important opportunity to achieve the European targets, especially considering that most of biogas plants are not fed with only dedicated biomass but also with residual biomass (typically manure), which can significantly improve the overall impact. Acknowledgements The authors would like to acknowledge Agricola Iraci Borgia company for assisting with data collection. References [1] Directive 2009/28/EC. On the promotion of the use of energy from renewable sources and amending and subsequently repealing Directives 2001/77/EC and 2003/30/EC.2009.04.23.Off J EU 2009.L 140:16-62. [2] Italian Ministry for Economic Development. Italy National Renewable Energy Action Plan; 2010.
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