the management of organic waste for the production of a
THE MANAGEMENT OF ORGANIC WASTE FOR THE PRODUCTION OF A FUEL FOR THE TRANSPORT SECTOR FILOMENA ARDOLINO*, FRANCESCO PARRILLO* AND UMBERTO ARENA* * Department of Environmental, Biological, Pharmaceutical Sciences and Technologies – University of Campania Luigi Vanvitelli, Via Vivaldi, 43, 81100 Caserta, ITALY
SUMMARY: The biowaste is an important fraction of solid waste, with a yearly generation rate of about 177 million tonnes only in the OECD countries. Its treatment by means of anaerobic digestion seems to have the best environmental and economic performances among all the possible biological treatments, allowing a minimization of greenhouse gas release, absence of emissions of bio-aerosols and bad odours, a limited land surface use, and a recovery of energy or fuels from a low cost biogas. In particular, there is a great interest in the possibility to use biofuels in the transport sector, due to the potential benefits of reduced emissions of pollutants into the atmosphere and diversification of transport fuel supplies. The study analyses and quantifies the improved overall sustainability of a biowaste anaerobic treatment where the produced biogas is upgraded to biomethane for the transport sector instead that directly burned in a combined heat and power unit. The avoided impacts related to the utilization of biomethane instead of diesel, petrol or natural gas have been evaluated with reference to a vehicle fleet made of passenger cars and small rigid trucks. They appear sufficiently large to make the biomethane production the cleanest option for the management of biowaste. An attributional life cycle assessment confirms this conclusion, by comparing the environmental performances of alternative configurations of an anaerobic digestion plant able to treat 100 t/d of biowaste. A sensitivity analysis evaluates the effect of several key parameters. Some of them are peculiar for the analysed application, such as the composition of the vehicle fleet, and methane slip in the biogas upgrading unit. Some other parameters are more general, and mainly related to the biological process, such as the final destination of solid digestate, national electric energy mix.
1. INTRODUCTION The biowaste typically includes food and kitchen waste from households and restaurants, waste from food processing plants, and biodegradable garden and park waste. It is an important fraction of total solid waste, and a crucial part of municipal solid waste (MSW). In the 35 OECD countries, which generate 44% of the total MSW of the world, this fraction varies significantly, from 14% to 56% of total MSW, with an average of 27% and a yearly generation rate of about 177 million tonnes (OECD, 2015). Only a limited part of this amount (37% in OECD countries, i.e. 66 million tonnes) is currently sent to biological treatments of aerobic (composting) and anaerobic digestion (ISWA, 2015). This indicates a huge potential of resource recovery from this biowaste: assuming an overall capture rate of 70%, it can be estimated that an additional amount of about 58 million tonnes/year could be sent to resource recovery, generating added value products having various levels of appealing. An essential taxonomy incudes (ISWA, Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
2015): - High value (and low volume and biomethane) products, such as the bio-based fine and specialty chemicals, generally used in limited quantities for high-technology applications (van Haveren et al., 2008; Fava et al., 2015); - Biofuels (such as biogas), bioplastics, cellulose, and commodity chemicals, which can be classified as medium value (and medium volume) products (Sheldon, 2011; Gallezot, 2012; Tuck et al., 2012); - The well-known compost and solid digestate, produced through aerobic and anaerobic digestion processes, respectively, which are considered as low value (and high volume) products (IEA Bioenergy, 2010). These simple observations give an idea of the importance of the global market for these products, and of a clean and economically feasible production process (Tock, 2017). The techno-scientific literature indicates that anaerobic digestion (AD) has the best environmental and economic performances among the possible biological treatments of the organic fraction of MSW (Mata-Alvarez, 2003; IEA-Bioenergy, 2005; Angelidaki and Batstone, 2011; Hermann et al., 2011; Lombardi et al., 2015; Tock, 2017). It allows a minimization of greenhouse gas (GHG) generation (Møller et al., 2009; Yoshida et al., 2012), absence of emissions of bio-aerosols and bad odours (Mata-Alvarez, 2000), a reduced land surface use (Zhang et al., 2007), and recovery of energy (Khalid et al. 2011; Evangelisti et al., 2014) or fuels (Pertl et al., 2010; Li et al., 2011; Bauer et al., 2013) from a low cost biogas. There are more than 17,000 biogas plants in Europe (mainly in Germany, with more than 50% of the total production, and in UK and Italy, with about 14% each), and about 1000 of them are fed with biowaste from MSW (Torrijos, 2016). The produced biogas is then cleaned to be appropriately used as gaseous fuel in generators and combined heat and power (CHP) systems, producing heat and electricity for internal consumptions of the AD plant and injecting the exess of electricity in the national grid (IEA-Bioenergy, 2005; Angelidaki and Batstone, 2011). Alternatively, it is treated to remove the non-methane components, i.e. it is upgraded to biomethane (Pertl et al., 2010; Bauer et al., 2013) for different applications, mainly for cooking domestic heating and fuel for shipping and road transport sectors (Patterson et al., 2011; Ricardo-AEA, 2016). In particular, there is a great interest in the utilisation of biomethane in the transport sector, due to the potential benefits related to the reduced emissions of GHGs and other pollutants into the atmosphere as well as the diversification of transport fuel supplies (Ricardo-AEA, 2015). This study aims to investigate how to improve the overall sustainability of the anaerobic treatment of biowaste, by analysing, the utilization of the obtained biogas for biomethane production, and quantifying the entity of the related avoided burdens from a life cycle perspective 2. METHODOLOGY The LCA follows the guidelines of the ISO standards (ISO, 2006a; 2006b) and utilises an attributional approach (Finnveden et. al, 2009) together with the software package SimaPro© 8.2. The intended application of the analysis is a reliable assessment of the environmental performances of an AD plant, equipped with a membrane separation unit able to upgrade the raw syngas and produce biomethane. These performances have been compared in a life cycle perspective with those of some alternative plant configurations, having a similar high technological reliability, and being able to satisfy the same functional unit. The analysed product system is the biomethane plant mentioned above, which includes: a wet anaerobic digester operating continuously under mesophilic regime at 37-39°C, and producing 400 m3N/h of raw biogas from 100 t/d of organic waste; a CHP unit, providing the energy for
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most of internal consumptions from the combustion of a limited part of the produced biogas; and a membrane separation unit, upgrading the remaining flow rate of raw biogas (3,500,000 m3N/y) and injecting 207 m3N/h in the methane grid. The study estimated mass and energy flows for the base case and some alternative plant configurations, and identified and quantified all the direct, indirect and avoided burdens. Figure 1 reports the quantified flow sheet related to the base case configuration, together with the codes of the European Waste Catalogue (EWC) for the different waste streams.
Figure 1. Quantified flow sheet of the biowaste-to-biomethane plant under analysis (base case), together with the EWC codes. Data are expressed in t/d and refer to the functional unit.
The functional unit coincides with the service provided by the plant, i.e. the treatment of 100 t/d of waste organic fraction obtained by MSW separate collection (Arena and Di Gregorio, 2014). The system boundaries, sketched in Figure 2 highlighting the foreground and background systems (Clift et al., 2000), include all the activities from the delivery of the biowaste at the plant entry gate until to the management of all process products (e.g., biomethane as transportation fuel) and residues. The allocation problem of the analysed systems has been avoided by utilizing the system expansion methodology (known as "avoided burden method"), by identifying which products are replaced on the markets by the obtained co-products and including their replacement in the model (Clift et al., 2000; Finnveden et al., 2009). In particular, the avoided impacts related to the utilisation of biomethane instead of diesel have been evaluated on data of a recent report about the role of biomethane in the transport sector (Ricardo-AEA, 2016), and the approach proposed by Vadenbo et al. (2017). It has been assumed that the produced biomethane fed a vehicle fleet including 50% of passenger cars and 50% of small rigid trucks moving on urban roads. The quality of data is high. All those related to the processes affected directly by decisions based on the study (i.e., the foreground system) derive from official data or measurements related to an existing AD plant, located in Italy and equipped with a CHP engine (Piancatelli, 2017), and to a biogas upgrading unit, utilising a membrane separation technology (Scholz et al., 2015; Barbato, 2017).
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Data related to the remaining, mainly indirect and avoided, burdens derive from the Ecoinvent 3.2 databank, technical reports and studies recently published on scientific literature (RicardoAEA, 2016; Anderson, 2015). In particular, those related to the avoided electricity production have been based on data of Italian electric energy mix, provided by International Energy Agency statistics (IEA, 2017). The study assesses the life cycle environmental impacts by means of the Impact 2002+, v2.11 methodology (Jolliet et al., 2003). BACKGROUND
ORGANIC WET FRACTION of MSW EWC 20.01.08 FOREGROUND Water and air emissions
Water Energy Chemicals
Anaerobic Digestion
Transportation
Solid residues EWC 19.12.12
Solid digestate
Landfill disposal
EWC 19.06.04
Biogas
EWC 19.06.99
Biogas upgrading unit
Internal combustion engine
Biomethane
Electricity
Production and utilization as automotive fuel
Electricity from grid
Figure 2. System boundaries for all the AD configurations under analysis, together with the indication of the foreground and background systems and the codes of European Waste Catalogue (EWC). Dashed lines refer to the solid streams or the avoided burdens that are present only in some of the analysed configurations.
The study collected all the data, whether measured, calculated or estimated, necessary to quantify the inputs and outputs of each unit process included within the system boundary (Figure 2), and then to construct the inventory table for each of the analysed plant configuration. An essential but exhaustive description of the unit processes included in the base case configuration is reported in the following, with reference to the flow sheet of Figure 1. The biowaste-to-biomethane plant is made-up of three main process units: pre-treatment, wet anaerobic digestion, and raw biogas upgrading unit. The pre-treatment is a mechanical sorting process that removes the out-of-target material, making the organic fraction from MSW (OFMSW) a substrate suitable for the anaerobic digestion process. The generated solid residues, which are 15.3% of the total waste inlet flow rate, are mainly made of dirty plastics, and are disposed in a close landfill. The wet anaerobic digester, which operates under mesophilic regime at 3739°C, produces a digestate and a biogas. The raw digestate is sent to a dehydration and dewatering process that separates the liquid fraction from the solid residues. The liquid digestate is treated on-site by using 0.5 t/d of sulfuric acid, and then discharged: the treated water mass flow rate is 13,000 tonnes/y with the following average pollutant concentrations: COD= 127 mg/kgwater, BOD5 = 40 mg/kgwater, suspended solids = 8 mg/kgwater, NH4= 5 mg/kgwater.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
All the values are below the limits imposed by Italian legislation (D. Lgs 152, 2006). The dried solid digestate is not suitable for agronomic utilization, as imposed by the Italian legislation (DM 5046, 2016), and it is disposed in a close landfill as stabilized material (DM 270910, 2010; Cavinato et al. 2013; Zeshan et al. 2014; Ciuta et al., 2016). The produced raw biogas, composed of 50.8% of methane and 44.6% of carbon dioxide, amounts to 140 m3N/twaste, i.e. 513 m3N/tVS with a degradation rate of the volatile solids of 63%, in agreement with data from scientific literature (Evangelisti et al. 2014; Hodge et al., 2016; Levis and Barlaz, 2011; Moller et al. 2009). In the biowaste-to-biomethane plant under analysis (base case scenario), a flow rate of 400 m3N/h of raw biogas is sent to the membrane upgrading unit. A combined heat and power (CHP) system burns the remaining part of raw biogas, 183 m3N/h, with an electric energy conversion efficiency of 38% and a production of 80.3 kWh/twaste. Self-consumptions of the plant are about 129 kWh/twaste, which are used for the operations of pre-treatment, anaerobic digestion and wastewater treatment (101 kWh/twaste), and for the operations of raw biogas upgrading (0.29 kWh for each normal cubic meter of raw biogas sent to upgrading, i.e. 28 kWh/twaste). The electricity from the Italian grid satisfies the remaining energy consumption. The recovered heat is completely utilised for the internal necessities of the plant. The biogas upgrading unit includes preliminary stages of biogas drying and compression, which is responsible of most of electrical consumptions, and hydrogen sulphide removal, which requires 3.4 t/y of activated carbon (i.e., about 1 g for each normal cubic meter of treated raw biogas). The upgrading unit is equipped with a high-efficiency three-stage membrane separation system made of polyimide hollow fibres (Chen et al., 2017), which provides a CO2 removal of 98.0% and a methane slip limited to 0.69% (Barbato, 2017). The base case configuration has three sources of emissions into atmosphere (Table 1): the CHP system, biofilter, and upgrading unit.The biofilter receives the gas stream coming from the areas of biowaste stock and pretreatment, and digestate processing, which is preliminary treated in a water scrubber, designed to operate in an environment with a low level of acidity, and able to remove dust, limit the picks of pollutant load, and moisturize the gas stream. Air emissions from the biofilter source derive from the values reported in the BREF document of European Union (JRC_EC, 2015). The emissions from biogas upgrading unit are the off-gas, composed mainly of the carbon neutral biogenic CO2 (98.9%), and only for a small part (0.79%) of CH4. Table 1 summarizes all the data reported above in terms of direct and avoided burdens, and it is the main part of the life cycle inventory table.
Table 1. Direct and avoided burdens for the scenarios under analysis. All the data refer to the functional unit. Base Scenario Scenario 1 Scenario 2 Scenario 3 Case study Only energy Only No energy biomethane from grid Biogas sent to CHP, m3N/h (%) 183 (31) 583 (100) - 283 (49) 3 Biogas sent to upgrading, m N/h (%) 400 (69) - 583 (100) 300 (51) DIRECT BURDENS Pre-treatment and anaerobic digestion phases Water (t) 1 1 1 1 Diesel (L) 30 30 30 30 Electrical energy from the grid (kWh) 2157 - 10100 - Solid residues to landfill, EWC 19.12.12 (t) 15.31 15.31 15.31 15.31 Solid digestate to landfill, EWC 19.06.04 15.60 15.60 15.60 15.60
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(t) CHP air emissions, tbiogas/d NOx (kg) CO (kg)
5.6 10.6 8.7
17.9 33.6 27.8
- - -
8.7 16.3 13.5
TOC (kg)
1.2
3.8
-
1.8
PM (kg) SO2 (kg) HCl (kg)
0.1 1.3 0.20
0.3 4.1 0.6
- - -
0.2 2.0 0.3
Hg (kg)
0.0003
0.001
0.0005
HF (kg)
0.02
0.06
-
0.03
NH3 (kg)
0.08
0.25
-
0.12
Cd (g)
0.8267
2.6
-
1.28
PCDD (mg)
0.0001
0.0004
-
0.0002
PAH (g)
2.9
9.1
-
4.4
Sb (g)
1.4
4.4
-
2.1
As (g)
1.4
4.4
-
2.1
Co (g)
1.4
4.4
-
2.1
Cr (g)
1.3
4.2
-
2.0
Mn (g)
2.1
6.8
-
3.3
Ni (g)
2.3
7.5
-
3.6
Pb (g)
1.4
4.4
-
2.1
Cu (g)
1.4
4.4
-
2.1
Vn (g)
1.4
4.4
-
2.1
CO2 biogenic (t)
8.2
26.2
-
12.7
607.2
607.2
607.2
607.2
Biofilter air emissions NH3 (g) Water emissions from liquid digestate treatment CODmax (kg) BOD5 (kg)
4.7 1.5
4.7 1.5
4.7 1.5
4.7 1.5
Suspended solids (kg)
0.30
0.30
0.30
0.30
NH4 (kg)
0.18
0.18
0.18
0.18
12.3
-
17.9
9.2
9.71 11.43 2784 24 8235 54550
- - - - - -
14.2 16.7 4060 35 12009 79550
7.29 8.57 - 18 6176 40910
Biogas upgrading unit, ttreated biogas/d Activated carbon consumption (kg) Activated carbon disposal (kg) Electrical energy from the grid (kWh) Air emissions (off-gas) CH4, biogenic (kg) CO2, biogenic (kg) Biomethane utilization Biomethane utilization in passenger car
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(km) Biomethane utilization in small rigid truck (km) AVOIDED BURDENS Electrical energy exported to the grid (kWh) Diesel production and utilization in passenger car (km) Diesel production and utilization in small rigid truck (km)
11190
-
16330
8400
-
15400
-
-
54550
-
79550
40910
11190
-
16330
8400
Fuel consumptions and the specific air emissions of assumed vehicle fleet are listed in Table 2. The obtained biomethane can be assumed all marketable since the Italian legislation provides economic incentives only for a demonstrated utilisation as transportation fuel of the produced biomethane. This means that a tonne of separately collected organic fraction of MSW leads to 49.7 m3N of biomethane, which in turn allow covering 112 km by small truck/bus and 546 km by passenger cars.
Table 2. Fuel consumptions of passenger cars and small rigid trucks with the specific air emissions utilized for the life cycle assessment.(Ricardo,2016; Anderson,2015;Ecoinvent, 2016) Biomethane Diesel
Passenger car
Small truck
Passenger car
m3N/100 km
Fuel consumption
4.56
Small truck
kg/100 km 22.2
Air emissions from fuel utilization
2.62
13.1
mg/km
CO2 fossil
-
-
1.07E+05
6.00E+05
CO biogenic
1290
350
-
-
-
-
0.60
3.0
CH4 biogenic
27.6
1250
-
-
N2O
0.64
109
4.0
4.0
NMVOC
50
-
13.2
17.5
Hydrocarbons
440
4840
1.34
1.66
NOx
59
291
210
291
SO2
0
0
0.46
1.91
0.3
1.6
1.5
16.1
-
-
0.16
0.20
CH4 fossil
PM2.5 Heavy metals
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Aldehydes and ketones
-
-
6.83
8.95
3. RESULTS AND DISCUSSION The normalised results of Life Cycle Impact Assessment reported in Figure 3 indicate that the impact categories that play a key role in the environmental performance of the system under analysis are those of GWP, NREP (non-renewable energy potential), RINP (respiratory inorganics potential), and, to a lesser extent, TECP (terrestrial ecotoxicity potential). It is noteworthy that all the total values for each impact category are negative (for GWP and NREP) or about zero, highlighting that the examined biowaste-to-biomethane plant implies a substantial reduction of the overall environmental impact. In other words, the avoided burdens related to the biomethane production and utilisation are larger than the direct and indirect burdens. The same figure also quantifies the specific contributions of the different stages of the life cycle. Large part of the avoided impacts derives from the missed production of diesel (“from crude oil to diesel”) and avoided “tank-to-wheels” (TTW) emissions for its utilisation in passenger cars and small rigid trucks (E4Tech, 2014; Ricardo-AEA, 2016). On the other hand, the corresponding contributions for the biowaste-to-biomethane production and biomethane utilisation in the same vehicles appear comparatively limited. The first are mainly related to the electricity consumptions (for the AD process and biogas upgrading) and to air emissions from the CHP. The contributions related to the utilisation of biomethane as transportation fuel are generally limited, and mainly related to the emissions of particulates and nitrous oxide.
Figure 3. Normalised results of impact assessment for the biowaste-to-biomethane plant under analysis (base case scenario). The shaded symbols indicate the total value for each impact category.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 4 reports the result of the comparative analyses between the four scenarios described in the Goal and Scope Definition, and synthetically identified by data listed in the first rows of Table 1. The plant configurations aimed to biomethane production (base scenario and scenario 2 and 3) have the best environmental performances. In particular, GWP and NREP values in the base scenario are better than they are in scenario 1 (“only energy”), with an improvement of 79% and 36%, respectively. Moreover, the results related to the impact category of Respiratory inorganics are enough close to each other, even though the specific contributions have a different origin. 0.20
0.00
-0.20
person · year
-0.40
-0.60
-0.80
-1.00
Base scenario Scenario 1
-1.20
Scenario 2 Scenario 3
-1.40
Respiratory inorganics
Terrestrial ecotoxicity
Global warming
Non-renewable energy
Figure 4. Normalized results of impact assessment for the four considered scenarios.
In this stage, a sensitivity analysis investigates how the variation of some key parameters affects the results of the LCIA. Some of the parameters taken into account are peculiar for the analysed application, such as the composition of the vehicle fleet (percentage of passenger cars and small rigid trucks), utilisation of another transportation fuel instead of diesel as reference fuel in the comparative analysis, specific biomethane consumptions, and methane slip in the upgrading unit. The other considered parameters are more general, such as the final destination of solid digestate, gas engine efficiency, and national electric energy mix. Figure 5 shows the normalized results for the three main impact categories as a function of different compositions of the vehicle fleet (from “100% passenger cars and 0% small trucks” to “0% passenger cars and 100% small trucks”). The examined variation of this parameter
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
confirms that the scenarios with biomethane production are always better than that of “only energy” production, mainly in terms of GWP and NREP. The parameter affects to a limited extent the Non-renewable energy but remarkably more the categories of Global warming and Respiratory inorganics. In particular, the RINP remains in a range of low values, even though it improves (i.e. its value decreases) when more passenger cars compose the vehicle fleet, reaching the minimum value when no small truck is in the fleet. This indicates that it is affected by the longer distances (and related avoided emissions) covered by biomethane cars, which are 1091 km/twaste in the case of 100% passenger cars, instead of 657 km/twaste in the base case scenario (545 km/twaste by cars and 112 km/twaste by trucks).
Figure 5. Normalized results of life cycle assessment as a function of different compositions of the vehicle fleet (percentage of passenger cars and small rigid trucks) for the base case scenario, scenario 1 (“only energy”), scenario 2 (“only biomethane”) and scenario 3 (“no energy from grid”).
The gradual improving of RINP with the gradual increase of biomethane production (scenario 3, base case scenario and scenario 2) confirms this positive effect. On the other hand, GWP improves when more small trucks are present in the vehicle fleet, due to the avoided greenhouse gas emissions (Table 2), which in the diesel small trucks are strongly higher than in diesel passenger cars (600 g/km vs 107 g/km).
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Table 3. Sensitivity analysis for all the considered scenarios as a function of some parameters. Reference Value Specific sensitivity parameters Higher methane slip in the upgrading (1%) Biomethane replaces fossil methane as transportation fuel Biomethane replaces petrol as pass. car fuel General sensitivity parameters Solid digestate to postcomposting Swedish electric energy mix French electric energy mix
Base case
RINP, kg PM2.5 Scen. Scen. 1 2
GWP, kg CO2 eq. Scen. 1 Scen. 2
Base case
-1.08E+04 -6.03E+03 -1.27E+04 -9.51E+03 -1.32E+05 -9.72E+04 -1.45E+05
0.11
0.18
0.20
0.32
0.12
-
0.07
0.33
-1.07E+04
-
0.97
-
1.31
0.89
-1.02E+04
-0.51
-
-0.84
-0.21
0.86
0.93
0.85
1.02
-1.26 -1.20
4.46 4.29
-3.88 -3.72
- -
Scen. 3
-1.28E+04 -9.44E+03 -1.32E+05
-
-1.48E+05
-
-1.21E+04 -9.23E+03 -1.92E+05
-
-2.36E+05
-1.83E+04
-
-2.40E+04 -1.53E+04 -2.92E+05
-
-3.82E+05
Base case
NREP, MJ primary Scen. 1 Scen. 2
Scen. 3
-1.01E+04 -5.57E+03 -1.22E+04 -8.98E+03 -1.24E+05 -9.12E+04 -1.39E+05 -1.28E+04 4.03E+02 -1.89E+04 -1.26E+04 -1.52E+02 -1.84E+04
- -
-1.39E+05 -7.75E+04 -1.67E+05 -1.05E+05 -1.82E+05 -7.05E+04
Scen. 3 1.23E+05 1.22E+05 1.69E+05 2.45E+05 1.15E+05
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Table 3 lists the values of all the other parameters of the sensitivity analysis and reports the characterisation values for RINP, GWP and NREP. There are limited effects for some of these parameters, such as the methane slip from the upgrading unit (varied from 0.69% to 1%, with the related lower biomethane production, and the fate of solid digestate (sent to composting instead of landfilling). The latter considers an on-site treatment in bio cells, producing a compost that substitutes peat in horticulture and/or hobby gardening, but requires about 69 kWh for each tonne of treated solid digestate (JRC_EC, 2014; 2015. Table 4. National electricity production mixes utilised in the sensitivity analysis. (Source: IEA, 2017) Italy Sweden France Non-renewable sources; % 61 36 84 Oil 5 - - Coal 16 1 2 Natural gas 40 - 4 Nuclear 0 35 78 Renewable sources, % 39 64 16 Hydroelectric 16 47 10 Biofuel and waste 8 7 1 Solar, wind and geothermal 15 10 5 The utilisation of a fuel different from diesel in the comparative analysis shows various effects. When biomethane replaces fossil natural gas as transportation fuel for cars and trucks, the biomethane scenarios show no variation of GWP, an improvement of NREP, and a worsening of RINP (the latter being anyway small in terms of person*year). On the contrary, when biomethane replaces petrol, the biomethane scenarios greatly improve their performances, thanks to the higher avoided impacts related to the petrol production and utilization. The variation of the national electric energy mix leads to strong variations for all the scenarios with the exclusion of scenario 3 “no energy from grid”. The base case refers to the Italian electricity production, which is made of a rather balanced mix of 61% of non-renewable sources (oil 5%, coal 16%, and natural gas 40%) and 39% of renewable sources (hydroelectric 16%, biofuels and waste 8%, solar, wind and geothermal 15%), as reported by the International Energy Agency (IEA, 2017). The analysis considered to completely different mixes, those of Sweden and France (Table 4), which imply a lower utilisation of non-renewable sources and a predominant role of nuclear source, respectively. Anyway, both have a lower utilization of fossil energy sources (1% for Sweden and 6% for France, against 61% for Italy): this implies lower avoided burdens related to the exported electric energy. The scenario 1 (“only energy”) is negatively affected by this variation, with results that appear particularly worse for RINP (from 0.18 kg PM2.5 to more than 4 kg PM2.5). For the same scenario, NREP improves with the French energy mix (from -9.72E+04 MJ primary to -18.2E+04 MJ primary), due to larger avoided burdens, and worsens with the Swedish mix (from -9.72E+04 MJ primary to -7.75E+04 MJ primary), due to smaller avoided burdens. The two scenarios of biomethane that utilize electricity from grid show improved RINP (for instance, in the base case, from 0.11 kg PM2.5 to about -1.20 kg PM2.5) and GWP (for instance, in the base case, from -1.1e+04 kg CO2 eq. to about -1.3E+04 kg CO2 eq.), as a direct consequence of the lower utilization of fossil fuels. NREP varies slightly but differently for the two alternatives, showing an improvement with the Swedish mix (due to the larger utilization of renewable sources) and a worsening with the French mix (due to the huge utilization of nuclear source).
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4. CONCLUSIONS The study assessed the sustainability of a process of anaerobic digestion of 100 t/d of organic fraction of a municipal solid waste, where the produced biogas is upgraded to biomethane for the transport sector by means of a membrane separation unit. An attributional life cycle assessment, developed for some alternative plant configurations, indicated that the biomethane production is always the cleanest option for the management of the analysed biowaste. A higher biomethane production leads always to higher avoided impacts, which largely balance all the other direct and indirect impacts. The quantification of the (generally large) avoided impacts related to the utilization of biomethane instead of diesel, petrol or fossil natural gas was referred to a vehicle fleet, made of passenger cars and small rigid trucks, in different percentages. Large part of the avoided impacts derives from the missed production of diesel and avoided “tank-to-wheels” emissions for its utilisation, while the corresponding contributions for the biowaste-to-biomethane production and biomethane utilisation in the same vehicles are rather limited. In particular, the biomethane scenarios improve the impact categories of global warming and non-renewable energy in the ranges 58-110% and 26-49%, respectively, with reference to the scenario with only energy production from the produced biogas. Some parameters could affect the results of the life cycle impact assessment. A different composition of the vehicle fleet does not turn the assessed better sustainability of the configurations with biomethane production. Anyway, a fleet with more passenger cars has lower values of respiratory inorganics potential, while a fleet with more small trucks has lower values of global warming potential. The utilisation of a fuel different from diesel in the comparative analysis shows various effects. When the substituted transportation fuel is the fossil natural gas, the biomethane scenarios show an improvement in the impact category of non-renewable energy, and a worsening in the (anyway low) values of respiratory inorganics. On the contrary, when biomethane replaces petrol, the biomethane scenarios have better performances, with large improvement of global warming and non-renewable energy potentials (up to 170%), thanks to the higher avoided impacts related to the petrol production and utilization.
ACKNOWLEDGMENTS The authors gratefully acknowledge Foglia Umberto s.r.l. and, in particular, Marco Piancatelli, which supported the study by providing the official measurements related to the anaerobic digestion plant and the technical information of all the components of the facility.
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Bauer F., Hulteberg C., Persson T., Tamm D. (2013). Biogas upgrading – Review of commercial technologies. SGC Rapport 2013:270, available at: http://vav.griffel.net/filer/C_SGC2013270.pdf Cavinato C., Bolzonella D., Pavan P., Fatone F., Cecchi F. (2013). Mesophilic and thermophilic anaerobic co-digestion of waste activated sludge and source sorted biowaste in pilot- and fullscale reactors. Renew. Energ., vol.55, 260-265 Chen X.Y., Kaliaguine S., Rodrigue D. (2017). A Comparison between Several Commercial Polymer Hollow Fiber Membranes for Gas Separation. J. Memb. Separ.Tech., vol.6, 1-15 Ciuta S., Antognoni S., Rada E.C., Ragazzi M., Badea A., Cioca L.L. (2016). Respirometric Index and Biogas Potential of Different Foods and Agricultural Discarded Biomass. Sustainability, 8-1311; doi:10.3390/su8121311 D. Lgs 152, 2006 (in Italian). Norme in materia ambientale. Italian Official Gazette n. 88-14 Avril 2006 – SO n. 96, annex 5, table III DM 27-09-10, 2010, (in Italian) Definizione dei criteri di ammissibilità dei rifiuti in discarica, in sostituzione di quelli contenuti nel decreto del Ministro dell'ambiente e della tutela del territorio 3 agosto 2005. Italian Official Gazette n. 281- 1 December 2010 DM 5046, 2016, (in Italian) Criteri e norme tecniche generali per la disciplina regionale dell'utilizzazione agronomica degli effluenti di allevamento e delle acque reflue, nonché per la produzione e l'utilizzazione agronomica del digestato. Italian Official Gazette n. 90-25 February 2016, Clause: 21-26 E4Tech, 2014. Advanced Biofuel Feedstocks – An Assessment of Sustainability. Framework for Transport-Related Technical and Engineering Advice and Research (PPRO 04/45/12). Available at: www.gov.uk/government/uploads/system/uploads/attachment_data/file/277436/feedstocksustainability.pdf Evangelisti S., Lettieri P., Borello D., Clift R. (2014). Life cycle assessment of energy from waste via anaerobic digestion: A UK case study. Waste Manage., vol.34, 226-237 Fava F., Totaro G., Diels L., Reis M., Duarte J., Beserra Carioca O., Poggi-Varaldo H.M., Sommer Ferreira B. (2015). Biowaste biorefinery in Europe: opportunities and research & development needs. N. Biotechnol., vol.32/1, 100-108; doi.org/10.1016/j.nbt.2013.11.003 Finnveden G., Hauschild M.Z., Ekvall T., Guinée J., Heijungs R., Hellweg S., Koehler A., Pennington D., Suh S. (2009). Recent developments in life cycle assessment. J. Environ. Manage. vol.91, 1–21. Gallezot P. (2012). Conversion of biomass to selected chemical products. Chem. Soc. Rev. 41:1538; doi:10.1039/c1cs15147a Hermann B.G., Debeer L., De Wilde B., Blok K., Patel M.K. (2011). To compost or not to compost: Carbon and energy footprints of biodegradable materials’ waste treatment. Polym. Degrad. Stabil., vol.96, 1159-1171 IEA. (2017). International Energy Agency. Energy statistics. Available at: https://www.iea.org/media/countries/Italy.pdf; https://www.iea.org/media/countries/france.pdf; https://www.iea.org/media/countries/sweden.pdf IEA-Bioenergy. (2005). Biogas Production and Utilisation. Report T37:2005:01. Available at: http://www.ieabioenergy.com/wp-content/uploads/2013/10/56_Task37booklet.pdf IEA-Bioenergy. (2010). Utilisation of digestate from biogas plants as biofertiliser. Report IEA Bioenergy Task 37. Available at: http://americanbiogascouncil.org/adCoProductsResources/IEA_digestate_Use_manual.pdf ISO. (2006a). Environmental Management-Life Cycle Assessment-Principles and Framework,
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2nd ed.; ISO 14040; 2006-07-01; ISO: Geneva, 2006 ISO. (2006b). Environmental Management-Life Cycle Assessment-Requirements Guidelines, 1st ed.; ISO 14044; 2006-07-01; ISO: Geneva, 2006
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ISWA. (2015). Circular Economy. Carbon, Nutrients and Soil. Report 4. Available at: www.iswa.org/iswa/iswagroups/task-forces/ Jolliet O., Margni M., Charles R., Humbert S., Payet J., Rebitzer G., Rosenbaum R. (2003). IMPACT 2002+: a new life cycle impact assessment methodology. Int. J.LCA, vol.8, 324–330. JRC_EC. (2015). Best Available Techniques (BAT) Reference Document for Waste Treatment. Joint Research Centre of the European Commission-Institute for Prospective Technological Studies Sustainable Production and Consumption Unit European IPPC Bureau. Draft 1, December 2015 JRC_EC. (2014). End-of-waste criteria for biodegradable waste subjected to biological treatment (compost & digestate): Technical proposals. Joint Research Centre of the European Commission. Available at: http://ftp.jrc.es/EURdoc/JRC87124.pdf Khalid A., Muhammad M., Anjum M., Mahmood T., Dawson L. (2011). The anaerobic digestion of solid organic waste. Review. Waste Manage., vol.31, 1737-1744 Levis J.W., and Barlaz M.A., 2011. What Is the Most Environmentally Beneficial Way to Treat Commercial Food Waste? Environ. Sci. Technol., vol.45, 7438-7444 Li Y., Park S.Y., Zhu J., (2011). Solid-state anaerobic digestion for methane production from organic waste. Ren. Sust. En. Rev., vol.15, 821-826 Lombardi L., Carnevale E.A., Corti A. (2015). Comparison of different biological treatment scenarios for the organic fraction of municipal solid waste, Int. J. Env. Sci. Tech., vol.12/1, 1-14 Mata-Alvarez J., Macée S., Llabrées P., (2000). Anaerobic digestion of organic solid wastes. An overview of research, achievements and perspectives. Biores. Tech., vol.74, 3-16 Mata-Alvarez J. (ed.), (2003). Biomethanization of the organic fraction of municipal solid wastes. IWA Publishing, London (UK), ISBN: 1 900222 14 0 Møller J., Boldrin A., Christensen T.H., (2009). Anaerobic digestion and digestate use: accounting of greenhouse gases and global warming contribution, Waste Manage. & Res., vol.27, 813-824 Munoz R., Meier L., Diaz I., Jeison D. (2015). A review on the state-of-the-art of physical/chemical and biological technologies for biogas upgrading. Rev. Env. Sci. Biotech., vol.14, 727-759 OECD, 2015. Waste, http://stats.oecd.org/
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Patterson T., Esteves S., Dinsdale R., Guwy A. (2011). Life cycle assessment of biogas infrastructure options on a regional scale. Bioresource Tech., vol.102, 7313-7323 Pertl A., Mostbauer P., Obersteiner G. (2010). Climate balance of biogas upgrading systems. Waste Manage., vol.30, 92-99 Ravina M. and Genon G. (2015). Global and local emissions of a biogas plant considering the production of biomethane as an alternative end-use solution. J. Cleaner Pro., vol.102, 115-126. Ricardo-AEA, (2015). Biomethane for Transport from Landfill and Anaerobic Digestion. Report PPRO 04/91/63 for UK Department of Transport, Report No. ED60023, Ricardo-AEA, (2016). The role of natural gas and biomethane in the transport sector. Report for Transport and Environment, Report No. ED 61479 Sheldon R.A. (2011). Utilisation of biomass for sustainable fuels and chemicals: Molecules, methods and metrics. Catal. Today, vol.167, 3-13
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Scholz M., Melin T., Wessling M. (2015). Transforming biogas into biomethane using membrane technology. Ren. Sust. En. Rev., vol.17, 199-212 Tock L., (2017). Sustainable waste-to-value biogas plants for developing countries. Waste Manage., vol.64, 1-2 Torrijos M. (2016). State of Development of Biogas Production in Europe. Proc. Environ. Scienc., vol.35, 881 – 889 Tuck C.O., Perez E., Horvath I.T., Sheldon R.A., Poliakoff M. (2012). Valorization of Biomass: Deriving More Value from Waste Science, vol.337, 695-699 Vadenbo C., Hellweg S., Astrup F.T. (2017). Let’s Be Clear(er) about Substitution, J. Ind. Ecol. doi:10.1111/jiec.12519 van Haveren J., Scott E.L., Sanders J. (2008). Bulk chemicals from biomass. Biofuels Bioprod. Bioref., 2-41; doi:10.1002/bbb.43 Yoshida H., Gable J.J., Park J.K. (2012). Evaluation of organic waste diversion alternatives for greenhouse gas reduction. Res. Cons. Rec., vol.60, 1-9 Zhang R., El-Mashad H.M., Hartman K. Wang F. Liu G. Choate C. Gamble P. (2007). Characterization of food waste as feedstock for anaerobic digestion. Biores. Techn., vol.98, 929-935
SUMMARY: The biowaste is an important fraction of solid waste, with a yearly generation rate of about 177 million tonnes only in the OECD countries. Its treatment by means of anaerobic digestion seems to have the best environmental and economic performances among all the possible biological treatments, allowing a minimization of greenhouse gas release, absence of emissions of bio-aerosols and bad odours, a limited land surface use, and a recovery of energy or fuels from a low cost biogas. In particular, there is a great interest in the possibility to use biofuels in the transport sector, due to the potential benefits of reduced emissions of pollutants into the atmosphere and diversification of transport fuel supplies. The study analyses and quantifies the improved overall sustainability of a biowaste anaerobic treatment where the produced biogas is upgraded to biomethane for the transport sector instead that directly burned in a combined heat and power unit. The avoided impacts related to the utilization of biomethane instead of diesel, petrol or natural gas have been evaluated with reference to a vehicle fleet made of passenger cars and small rigid trucks. They appear sufficiently large to make the biomethane production the cleanest option for the management of biowaste. An attributional life cycle assessment confirms this conclusion, by comparing the environmental performances of alternative configurations of an anaerobic digestion plant able to treat 100 t/d of biowaste. A sensitivity analysis evaluates the effect of several key parameters. Some of them are peculiar for the analysed application, such as the composition of the vehicle fleet, and methane slip in the biogas upgrading unit. Some other parameters are more general, and mainly related to the biological process, such as the final destination of solid digestate, national electric energy mix.
1. INTRODUCTION The biowaste typically includes food and kitchen waste from households and restaurants, waste from food processing plants, and biodegradable garden and park waste. It is an important fraction of total solid waste, and a crucial part of municipal solid waste (MSW). In the 35 OECD countries, which generate 44% of the total MSW of the world, this fraction varies significantly, from 14% to 56% of total MSW, with an average of 27% and a yearly generation rate of about 177 million tonnes (OECD, 2015). Only a limited part of this amount (37% in OECD countries, i.e. 66 million tonnes) is currently sent to biological treatments of aerobic (composting) and anaerobic digestion (ISWA, 2015). This indicates a huge potential of resource recovery from this biowaste: assuming an overall capture rate of 70%, it can be estimated that an additional amount of about 58 million tonnes/year could be sent to resource recovery, generating added value products having various levels of appealing. An essential taxonomy incudes (ISWA, Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
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2015): - High value (and low volume and biomethane) products, such as the bio-based fine and specialty chemicals, generally used in limited quantities for high-technology applications (van Haveren et al., 2008; Fava et al., 2015); - Biofuels (such as biogas), bioplastics, cellulose, and commodity chemicals, which can be classified as medium value (and medium volume) products (Sheldon, 2011; Gallezot, 2012; Tuck et al., 2012); - The well-known compost and solid digestate, produced through aerobic and anaerobic digestion processes, respectively, which are considered as low value (and high volume) products (IEA Bioenergy, 2010). These simple observations give an idea of the importance of the global market for these products, and of a clean and economically feasible production process (Tock, 2017). The techno-scientific literature indicates that anaerobic digestion (AD) has the best environmental and economic performances among the possible biological treatments of the organic fraction of MSW (Mata-Alvarez, 2003; IEA-Bioenergy, 2005; Angelidaki and Batstone, 2011; Hermann et al., 2011; Lombardi et al., 2015; Tock, 2017). It allows a minimization of greenhouse gas (GHG) generation (Møller et al., 2009; Yoshida et al., 2012), absence of emissions of bio-aerosols and bad odours (Mata-Alvarez, 2000), a reduced land surface use (Zhang et al., 2007), and recovery of energy (Khalid et al. 2011; Evangelisti et al., 2014) or fuels (Pertl et al., 2010; Li et al., 2011; Bauer et al., 2013) from a low cost biogas. There are more than 17,000 biogas plants in Europe (mainly in Germany, with more than 50% of the total production, and in UK and Italy, with about 14% each), and about 1000 of them are fed with biowaste from MSW (Torrijos, 2016). The produced biogas is then cleaned to be appropriately used as gaseous fuel in generators and combined heat and power (CHP) systems, producing heat and electricity for internal consumptions of the AD plant and injecting the exess of electricity in the national grid (IEA-Bioenergy, 2005; Angelidaki and Batstone, 2011). Alternatively, it is treated to remove the non-methane components, i.e. it is upgraded to biomethane (Pertl et al., 2010; Bauer et al., 2013) for different applications, mainly for cooking domestic heating and fuel for shipping and road transport sectors (Patterson et al., 2011; Ricardo-AEA, 2016). In particular, there is a great interest in the utilisation of biomethane in the transport sector, due to the potential benefits related to the reduced emissions of GHGs and other pollutants into the atmosphere as well as the diversification of transport fuel supplies (Ricardo-AEA, 2015). This study aims to investigate how to improve the overall sustainability of the anaerobic treatment of biowaste, by analysing, the utilization of the obtained biogas for biomethane production, and quantifying the entity of the related avoided burdens from a life cycle perspective 2. METHODOLOGY The LCA follows the guidelines of the ISO standards (ISO, 2006a; 2006b) and utilises an attributional approach (Finnveden et. al, 2009) together with the software package SimaPro© 8.2. The intended application of the analysis is a reliable assessment of the environmental performances of an AD plant, equipped with a membrane separation unit able to upgrade the raw syngas and produce biomethane. These performances have been compared in a life cycle perspective with those of some alternative plant configurations, having a similar high technological reliability, and being able to satisfy the same functional unit. The analysed product system is the biomethane plant mentioned above, which includes: a wet anaerobic digester operating continuously under mesophilic regime at 37-39°C, and producing 400 m3N/h of raw biogas from 100 t/d of organic waste; a CHP unit, providing the energy for
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most of internal consumptions from the combustion of a limited part of the produced biogas; and a membrane separation unit, upgrading the remaining flow rate of raw biogas (3,500,000 m3N/y) and injecting 207 m3N/h in the methane grid. The study estimated mass and energy flows for the base case and some alternative plant configurations, and identified and quantified all the direct, indirect and avoided burdens. Figure 1 reports the quantified flow sheet related to the base case configuration, together with the codes of the European Waste Catalogue (EWC) for the different waste streams.
Figure 1. Quantified flow sheet of the biowaste-to-biomethane plant under analysis (base case), together with the EWC codes. Data are expressed in t/d and refer to the functional unit.
The functional unit coincides with the service provided by the plant, i.e. the treatment of 100 t/d of waste organic fraction obtained by MSW separate collection (Arena and Di Gregorio, 2014). The system boundaries, sketched in Figure 2 highlighting the foreground and background systems (Clift et al., 2000), include all the activities from the delivery of the biowaste at the plant entry gate until to the management of all process products (e.g., biomethane as transportation fuel) and residues. The allocation problem of the analysed systems has been avoided by utilizing the system expansion methodology (known as "avoided burden method"), by identifying which products are replaced on the markets by the obtained co-products and including their replacement in the model (Clift et al., 2000; Finnveden et al., 2009). In particular, the avoided impacts related to the utilisation of biomethane instead of diesel have been evaluated on data of a recent report about the role of biomethane in the transport sector (Ricardo-AEA, 2016), and the approach proposed by Vadenbo et al. (2017). It has been assumed that the produced biomethane fed a vehicle fleet including 50% of passenger cars and 50% of small rigid trucks moving on urban roads. The quality of data is high. All those related to the processes affected directly by decisions based on the study (i.e., the foreground system) derive from official data or measurements related to an existing AD plant, located in Italy and equipped with a CHP engine (Piancatelli, 2017), and to a biogas upgrading unit, utilising a membrane separation technology (Scholz et al., 2015; Barbato, 2017).
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Data related to the remaining, mainly indirect and avoided, burdens derive from the Ecoinvent 3.2 databank, technical reports and studies recently published on scientific literature (RicardoAEA, 2016; Anderson, 2015). In particular, those related to the avoided electricity production have been based on data of Italian electric energy mix, provided by International Energy Agency statistics (IEA, 2017). The study assesses the life cycle environmental impacts by means of the Impact 2002+, v2.11 methodology (Jolliet et al., 2003). BACKGROUND
ORGANIC WET FRACTION of MSW EWC 20.01.08 FOREGROUND Water and air emissions
Water Energy Chemicals
Anaerobic Digestion
Transportation
Solid residues EWC 19.12.12
Solid digestate
Landfill disposal
EWC 19.06.04
Biogas
EWC 19.06.99
Biogas upgrading unit
Internal combustion engine
Biomethane
Electricity
Production and utilization as automotive fuel
Electricity from grid
Figure 2. System boundaries for all the AD configurations under analysis, together with the indication of the foreground and background systems and the codes of European Waste Catalogue (EWC). Dashed lines refer to the solid streams or the avoided burdens that are present only in some of the analysed configurations.
The study collected all the data, whether measured, calculated or estimated, necessary to quantify the inputs and outputs of each unit process included within the system boundary (Figure 2), and then to construct the inventory table for each of the analysed plant configuration. An essential but exhaustive description of the unit processes included in the base case configuration is reported in the following, with reference to the flow sheet of Figure 1. The biowaste-to-biomethane plant is made-up of three main process units: pre-treatment, wet anaerobic digestion, and raw biogas upgrading unit. The pre-treatment is a mechanical sorting process that removes the out-of-target material, making the organic fraction from MSW (OFMSW) a substrate suitable for the anaerobic digestion process. The generated solid residues, which are 15.3% of the total waste inlet flow rate, are mainly made of dirty plastics, and are disposed in a close landfill. The wet anaerobic digester, which operates under mesophilic regime at 3739°C, produces a digestate and a biogas. The raw digestate is sent to a dehydration and dewatering process that separates the liquid fraction from the solid residues. The liquid digestate is treated on-site by using 0.5 t/d of sulfuric acid, and then discharged: the treated water mass flow rate is 13,000 tonnes/y with the following average pollutant concentrations: COD= 127 mg/kgwater, BOD5 = 40 mg/kgwater, suspended solids = 8 mg/kgwater, NH4= 5 mg/kgwater.
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All the values are below the limits imposed by Italian legislation (D. Lgs 152, 2006). The dried solid digestate is not suitable for agronomic utilization, as imposed by the Italian legislation (DM 5046, 2016), and it is disposed in a close landfill as stabilized material (DM 270910, 2010; Cavinato et al. 2013; Zeshan et al. 2014; Ciuta et al., 2016). The produced raw biogas, composed of 50.8% of methane and 44.6% of carbon dioxide, amounts to 140 m3N/twaste, i.e. 513 m3N/tVS with a degradation rate of the volatile solids of 63%, in agreement with data from scientific literature (Evangelisti et al. 2014; Hodge et al., 2016; Levis and Barlaz, 2011; Moller et al. 2009). In the biowaste-to-biomethane plant under analysis (base case scenario), a flow rate of 400 m3N/h of raw biogas is sent to the membrane upgrading unit. A combined heat and power (CHP) system burns the remaining part of raw biogas, 183 m3N/h, with an electric energy conversion efficiency of 38% and a production of 80.3 kWh/twaste. Self-consumptions of the plant are about 129 kWh/twaste, which are used for the operations of pre-treatment, anaerobic digestion and wastewater treatment (101 kWh/twaste), and for the operations of raw biogas upgrading (0.29 kWh for each normal cubic meter of raw biogas sent to upgrading, i.e. 28 kWh/twaste). The electricity from the Italian grid satisfies the remaining energy consumption. The recovered heat is completely utilised for the internal necessities of the plant. The biogas upgrading unit includes preliminary stages of biogas drying and compression, which is responsible of most of electrical consumptions, and hydrogen sulphide removal, which requires 3.4 t/y of activated carbon (i.e., about 1 g for each normal cubic meter of treated raw biogas). The upgrading unit is equipped with a high-efficiency three-stage membrane separation system made of polyimide hollow fibres (Chen et al., 2017), which provides a CO2 removal of 98.0% and a methane slip limited to 0.69% (Barbato, 2017). The base case configuration has three sources of emissions into atmosphere (Table 1): the CHP system, biofilter, and upgrading unit.The biofilter receives the gas stream coming from the areas of biowaste stock and pretreatment, and digestate processing, which is preliminary treated in a water scrubber, designed to operate in an environment with a low level of acidity, and able to remove dust, limit the picks of pollutant load, and moisturize the gas stream. Air emissions from the biofilter source derive from the values reported in the BREF document of European Union (JRC_EC, 2015). The emissions from biogas upgrading unit are the off-gas, composed mainly of the carbon neutral biogenic CO2 (98.9%), and only for a small part (0.79%) of CH4. Table 1 summarizes all the data reported above in terms of direct and avoided burdens, and it is the main part of the life cycle inventory table.
Table 1. Direct and avoided burdens for the scenarios under analysis. All the data refer to the functional unit. Base Scenario Scenario 1 Scenario 2 Scenario 3 Case study Only energy Only No energy biomethane from grid Biogas sent to CHP, m3N/h (%) 183 (31) 583 (100) - 283 (49) 3 Biogas sent to upgrading, m N/h (%) 400 (69) - 583 (100) 300 (51) DIRECT BURDENS Pre-treatment and anaerobic digestion phases Water (t) 1 1 1 1 Diesel (L) 30 30 30 30 Electrical energy from the grid (kWh) 2157 - 10100 - Solid residues to landfill, EWC 19.12.12 (t) 15.31 15.31 15.31 15.31 Solid digestate to landfill, EWC 19.06.04 15.60 15.60 15.60 15.60
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(t) CHP air emissions, tbiogas/d NOx (kg) CO (kg)
5.6 10.6 8.7
17.9 33.6 27.8
- - -
8.7 16.3 13.5
TOC (kg)
1.2
3.8
-
1.8
PM (kg) SO2 (kg) HCl (kg)
0.1 1.3 0.20
0.3 4.1 0.6
- - -
0.2 2.0 0.3
Hg (kg)
0.0003
0.001
0.0005
HF (kg)
0.02
0.06
-
0.03
NH3 (kg)
0.08
0.25
-
0.12
Cd (g)
0.8267
2.6
-
1.28
PCDD (mg)
0.0001
0.0004
-
0.0002
PAH (g)
2.9
9.1
-
4.4
Sb (g)
1.4
4.4
-
2.1
As (g)
1.4
4.4
-
2.1
Co (g)
1.4
4.4
-
2.1
Cr (g)
1.3
4.2
-
2.0
Mn (g)
2.1
6.8
-
3.3
Ni (g)
2.3
7.5
-
3.6
Pb (g)
1.4
4.4
-
2.1
Cu (g)
1.4
4.4
-
2.1
Vn (g)
1.4
4.4
-
2.1
CO2 biogenic (t)
8.2
26.2
-
12.7
607.2
607.2
607.2
607.2
Biofilter air emissions NH3 (g) Water emissions from liquid digestate treatment CODmax (kg) BOD5 (kg)
4.7 1.5
4.7 1.5
4.7 1.5
4.7 1.5
Suspended solids (kg)
0.30
0.30
0.30
0.30
NH4 (kg)
0.18
0.18
0.18
0.18
12.3
-
17.9
9.2
9.71 11.43 2784 24 8235 54550
- - - - - -
14.2 16.7 4060 35 12009 79550
7.29 8.57 - 18 6176 40910
Biogas upgrading unit, ttreated biogas/d Activated carbon consumption (kg) Activated carbon disposal (kg) Electrical energy from the grid (kWh) Air emissions (off-gas) CH4, biogenic (kg) CO2, biogenic (kg) Biomethane utilization Biomethane utilization in passenger car
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
(km) Biomethane utilization in small rigid truck (km) AVOIDED BURDENS Electrical energy exported to the grid (kWh) Diesel production and utilization in passenger car (km) Diesel production and utilization in small rigid truck (km)
11190
-
16330
8400
-
15400
-
-
54550
-
79550
40910
11190
-
16330
8400
Fuel consumptions and the specific air emissions of assumed vehicle fleet are listed in Table 2. The obtained biomethane can be assumed all marketable since the Italian legislation provides economic incentives only for a demonstrated utilisation as transportation fuel of the produced biomethane. This means that a tonne of separately collected organic fraction of MSW leads to 49.7 m3N of biomethane, which in turn allow covering 112 km by small truck/bus and 546 km by passenger cars.
Table 2. Fuel consumptions of passenger cars and small rigid trucks with the specific air emissions utilized for the life cycle assessment.(Ricardo,2016; Anderson,2015;Ecoinvent, 2016) Biomethane Diesel
Passenger car
Small truck
Passenger car
m3N/100 km
Fuel consumption
4.56
Small truck
kg/100 km 22.2
Air emissions from fuel utilization
2.62
13.1
mg/km
CO2 fossil
-
-
1.07E+05
6.00E+05
CO biogenic
1290
350
-
-
-
-
0.60
3.0
CH4 biogenic
27.6
1250
-
-
N2O
0.64
109
4.0
4.0
NMVOC
50
-
13.2
17.5
Hydrocarbons
440
4840
1.34
1.66
NOx
59
291
210
291
SO2
0
0
0.46
1.91
0.3
1.6
1.5
16.1
-
-
0.16
0.20
CH4 fossil
PM2.5 Heavy metals
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Aldehydes and ketones
-
-
6.83
8.95
3. RESULTS AND DISCUSSION The normalised results of Life Cycle Impact Assessment reported in Figure 3 indicate that the impact categories that play a key role in the environmental performance of the system under analysis are those of GWP, NREP (non-renewable energy potential), RINP (respiratory inorganics potential), and, to a lesser extent, TECP (terrestrial ecotoxicity potential). It is noteworthy that all the total values for each impact category are negative (for GWP and NREP) or about zero, highlighting that the examined biowaste-to-biomethane plant implies a substantial reduction of the overall environmental impact. In other words, the avoided burdens related to the biomethane production and utilisation are larger than the direct and indirect burdens. The same figure also quantifies the specific contributions of the different stages of the life cycle. Large part of the avoided impacts derives from the missed production of diesel (“from crude oil to diesel”) and avoided “tank-to-wheels” (TTW) emissions for its utilisation in passenger cars and small rigid trucks (E4Tech, 2014; Ricardo-AEA, 2016). On the other hand, the corresponding contributions for the biowaste-to-biomethane production and biomethane utilisation in the same vehicles appear comparatively limited. The first are mainly related to the electricity consumptions (for the AD process and biogas upgrading) and to air emissions from the CHP. The contributions related to the utilisation of biomethane as transportation fuel are generally limited, and mainly related to the emissions of particulates and nitrous oxide.
Figure 3. Normalised results of impact assessment for the biowaste-to-biomethane plant under analysis (base case scenario). The shaded symbols indicate the total value for each impact category.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 4 reports the result of the comparative analyses between the four scenarios described in the Goal and Scope Definition, and synthetically identified by data listed in the first rows of Table 1. The plant configurations aimed to biomethane production (base scenario and scenario 2 and 3) have the best environmental performances. In particular, GWP and NREP values in the base scenario are better than they are in scenario 1 (“only energy”), with an improvement of 79% and 36%, respectively. Moreover, the results related to the impact category of Respiratory inorganics are enough close to each other, even though the specific contributions have a different origin. 0.20
0.00
-0.20
person · year
-0.40
-0.60
-0.80
-1.00
Base scenario Scenario 1
-1.20
Scenario 2 Scenario 3
-1.40
Respiratory inorganics
Terrestrial ecotoxicity
Global warming
Non-renewable energy
Figure 4. Normalized results of impact assessment for the four considered scenarios.
In this stage, a sensitivity analysis investigates how the variation of some key parameters affects the results of the LCIA. Some of the parameters taken into account are peculiar for the analysed application, such as the composition of the vehicle fleet (percentage of passenger cars and small rigid trucks), utilisation of another transportation fuel instead of diesel as reference fuel in the comparative analysis, specific biomethane consumptions, and methane slip in the upgrading unit. The other considered parameters are more general, such as the final destination of solid digestate, gas engine efficiency, and national electric energy mix. Figure 5 shows the normalized results for the three main impact categories as a function of different compositions of the vehicle fleet (from “100% passenger cars and 0% small trucks” to “0% passenger cars and 100% small trucks”). The examined variation of this parameter
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
confirms that the scenarios with biomethane production are always better than that of “only energy” production, mainly in terms of GWP and NREP. The parameter affects to a limited extent the Non-renewable energy but remarkably more the categories of Global warming and Respiratory inorganics. In particular, the RINP remains in a range of low values, even though it improves (i.e. its value decreases) when more passenger cars compose the vehicle fleet, reaching the minimum value when no small truck is in the fleet. This indicates that it is affected by the longer distances (and related avoided emissions) covered by biomethane cars, which are 1091 km/twaste in the case of 100% passenger cars, instead of 657 km/twaste in the base case scenario (545 km/twaste by cars and 112 km/twaste by trucks).
Figure 5. Normalized results of life cycle assessment as a function of different compositions of the vehicle fleet (percentage of passenger cars and small rigid trucks) for the base case scenario, scenario 1 (“only energy”), scenario 2 (“only biomethane”) and scenario 3 (“no energy from grid”).
The gradual improving of RINP with the gradual increase of biomethane production (scenario 3, base case scenario and scenario 2) confirms this positive effect. On the other hand, GWP improves when more small trucks are present in the vehicle fleet, due to the avoided greenhouse gas emissions (Table 2), which in the diesel small trucks are strongly higher than in diesel passenger cars (600 g/km vs 107 g/km).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 3. Sensitivity analysis for all the considered scenarios as a function of some parameters. Reference Value Specific sensitivity parameters Higher methane slip in the upgrading (1%) Biomethane replaces fossil methane as transportation fuel Biomethane replaces petrol as pass. car fuel General sensitivity parameters Solid digestate to postcomposting Swedish electric energy mix French electric energy mix
Base case
RINP, kg PM2.5 Scen. Scen. 1 2
GWP, kg CO2 eq. Scen. 1 Scen. 2
Base case
-1.08E+04 -6.03E+03 -1.27E+04 -9.51E+03 -1.32E+05 -9.72E+04 -1.45E+05
0.11
0.18
0.20
0.32
0.12
-
0.07
0.33
-1.07E+04
-
0.97
-
1.31
0.89
-1.02E+04
-0.51
-
-0.84
-0.21
0.86
0.93
0.85
1.02
-1.26 -1.20
4.46 4.29
-3.88 -3.72
- -
Scen. 3
-1.28E+04 -9.44E+03 -1.32E+05
-
-1.48E+05
-
-1.21E+04 -9.23E+03 -1.92E+05
-
-2.36E+05
-1.83E+04
-
-2.40E+04 -1.53E+04 -2.92E+05
-
-3.82E+05
Base case
NREP, MJ primary Scen. 1 Scen. 2
Scen. 3
-1.01E+04 -5.57E+03 -1.22E+04 -8.98E+03 -1.24E+05 -9.12E+04 -1.39E+05 -1.28E+04 4.03E+02 -1.89E+04 -1.26E+04 -1.52E+02 -1.84E+04
- -
-1.39E+05 -7.75E+04 -1.67E+05 -1.05E+05 -1.82E+05 -7.05E+04
Scen. 3 1.23E+05 1.22E+05 1.69E+05 2.45E+05 1.15E+05
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 3 lists the values of all the other parameters of the sensitivity analysis and reports the characterisation values for RINP, GWP and NREP. There are limited effects for some of these parameters, such as the methane slip from the upgrading unit (varied from 0.69% to 1%, with the related lower biomethane production, and the fate of solid digestate (sent to composting instead of landfilling). The latter considers an on-site treatment in bio cells, producing a compost that substitutes peat in horticulture and/or hobby gardening, but requires about 69 kWh for each tonne of treated solid digestate (JRC_EC, 2014; 2015. Table 4. National electricity production mixes utilised in the sensitivity analysis. (Source: IEA, 2017) Italy Sweden France Non-renewable sources; % 61 36 84 Oil 5 - - Coal 16 1 2 Natural gas 40 - 4 Nuclear 0 35 78 Renewable sources, % 39 64 16 Hydroelectric 16 47 10 Biofuel and waste 8 7 1 Solar, wind and geothermal 15 10 5 The utilisation of a fuel different from diesel in the comparative analysis shows various effects. When biomethane replaces fossil natural gas as transportation fuel for cars and trucks, the biomethane scenarios show no variation of GWP, an improvement of NREP, and a worsening of RINP (the latter being anyway small in terms of person*year). On the contrary, when biomethane replaces petrol, the biomethane scenarios greatly improve their performances, thanks to the higher avoided impacts related to the petrol production and utilization. The variation of the national electric energy mix leads to strong variations for all the scenarios with the exclusion of scenario 3 “no energy from grid”. The base case refers to the Italian electricity production, which is made of a rather balanced mix of 61% of non-renewable sources (oil 5%, coal 16%, and natural gas 40%) and 39% of renewable sources (hydroelectric 16%, biofuels and waste 8%, solar, wind and geothermal 15%), as reported by the International Energy Agency (IEA, 2017). The analysis considered to completely different mixes, those of Sweden and France (Table 4), which imply a lower utilisation of non-renewable sources and a predominant role of nuclear source, respectively. Anyway, both have a lower utilization of fossil energy sources (1% for Sweden and 6% for France, against 61% for Italy): this implies lower avoided burdens related to the exported electric energy. The scenario 1 (“only energy”) is negatively affected by this variation, with results that appear particularly worse for RINP (from 0.18 kg PM2.5 to more than 4 kg PM2.5). For the same scenario, NREP improves with the French energy mix (from -9.72E+04 MJ primary to -18.2E+04 MJ primary), due to larger avoided burdens, and worsens with the Swedish mix (from -9.72E+04 MJ primary to -7.75E+04 MJ primary), due to smaller avoided burdens. The two scenarios of biomethane that utilize electricity from grid show improved RINP (for instance, in the base case, from 0.11 kg PM2.5 to about -1.20 kg PM2.5) and GWP (for instance, in the base case, from -1.1e+04 kg CO2 eq. to about -1.3E+04 kg CO2 eq.), as a direct consequence of the lower utilization of fossil fuels. NREP varies slightly but differently for the two alternatives, showing an improvement with the Swedish mix (due to the larger utilization of renewable sources) and a worsening with the French mix (due to the huge utilization of nuclear source).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
4. CONCLUSIONS The study assessed the sustainability of a process of anaerobic digestion of 100 t/d of organic fraction of a municipal solid waste, where the produced biogas is upgraded to biomethane for the transport sector by means of a membrane separation unit. An attributional life cycle assessment, developed for some alternative plant configurations, indicated that the biomethane production is always the cleanest option for the management of the analysed biowaste. A higher biomethane production leads always to higher avoided impacts, which largely balance all the other direct and indirect impacts. The quantification of the (generally large) avoided impacts related to the utilization of biomethane instead of diesel, petrol or fossil natural gas was referred to a vehicle fleet, made of passenger cars and small rigid trucks, in different percentages. Large part of the avoided impacts derives from the missed production of diesel and avoided “tank-to-wheels” emissions for its utilisation, while the corresponding contributions for the biowaste-to-biomethane production and biomethane utilisation in the same vehicles are rather limited. In particular, the biomethane scenarios improve the impact categories of global warming and non-renewable energy in the ranges 58-110% and 26-49%, respectively, with reference to the scenario with only energy production from the produced biogas. Some parameters could affect the results of the life cycle impact assessment. A different composition of the vehicle fleet does not turn the assessed better sustainability of the configurations with biomethane production. Anyway, a fleet with more passenger cars has lower values of respiratory inorganics potential, while a fleet with more small trucks has lower values of global warming potential. The utilisation of a fuel different from diesel in the comparative analysis shows various effects. When the substituted transportation fuel is the fossil natural gas, the biomethane scenarios show an improvement in the impact category of non-renewable energy, and a worsening in the (anyway low) values of respiratory inorganics. On the contrary, when biomethane replaces petrol, the biomethane scenarios have better performances, with large improvement of global warming and non-renewable energy potentials (up to 170%), thanks to the higher avoided impacts related to the petrol production and utilization.
ACKNOWLEDGMENTS The authors gratefully acknowledge Foglia Umberto s.r.l. and, in particular, Marco Piancatelli, which supported the study by providing the official measurements related to the anaerobic digestion plant and the technical information of all the components of the facility.
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