Carbon and energy footprint of the hydrate-based biogas upgrading process integrated with CO2 valorization

Science of the Total Environment 615 (2018) 404–411

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Carbon and energy footprint of the hydrate-based biogas upgrading process integrated with CO2 valorization Beatrice Castellani a,b, Sara Rinaldi a, Emanuele Bonamente a,b,⁎, Andrea Nicolini a,b, Federico Rossi a,b, Franco Cotana a,b a b

CIRIAF – Interuniversity Research Center on Pollution and Environment, University of Perugia, Via G. Duranti, Perugia, Italy Department of Engineering, University of Perugia, Via G. Duranti, Perugia, Italy

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• The hydrate-based upgrading process is integrated with the CO2 methanation process. • Carbon and energy footprint is evaluated from a cradle-to-gate perspective. • Carbon and energy footprint is 0.7081 kgCO2eq and 28.55 MJ per 1 Nm3 of biosynthetic methane. • Hydrogen production is the most impacting phase of the process. • The environmental footprint of the process is comparable to data for biomethane.

a r t i c l e

i n f o

Article history: Received 31 July 2017 Received in revised form 21 September 2017 Accepted 23 September 2017 Available online xxxx Editor: D. Barcelo Keywords: Carbon footprint Energy footprint Biogas upgrading Clathrate hydrates CO2 methanation LCA

a b s t r a c t The present paper aims at assessing the carbon and energy footprint of an energy process, in which the energy excess from intermittent renewable sources is used to produce hydrogen which reacts with the CO2 previously separated from an innovative biogas upgrading process. The process integrates a hydrate-based biogas upgrading section and a CO2 methanation section, to produce biomethane from the biogas enrichment and synthetic methane from the CO2 methanation. Clathrate hydrates are crystalline compounds, formed by gas enclathrated in cages of water molecules and are applied to the selective separation of CO2 from biogas mixtures. Data from the experimental setup were analyzed in order to evaluate the green-house gas emissions (carbon footprint CF) and the primary energy consumption (energy footprint EF) associated to the two sections of the process. The biosynthetic methane production during a single-stage process was 0.962 Nm3, obtained mixing 0.830 Nm3 of methane-enriched biogas and 0.132 Nm3 of synthetic methane. The final volume composition was: 73.82% CH4, 19.47% CO2, 0.67% H2, 1.98% O2, 4.06% N2 and the energy content was 28.0 MJ/Nm3. The functional unit is the unitary amount of produced biosynthetic methane in Nm3. Carbon and energy footprints are 0.7081 kgCO2eq/Nm3 and 28.55 MJ/Nm3, respectively, when the electric energy required by the process is provided by photovoltaic panels. In this scenario, the overall energy efficiency is about 0.82, higher than the worldwide average energy efficiency for fossil methane, which is 0.75. © 2017 Elsevier B.V. All rights reserved.

1. Introduction ⁎ Corresponding author at: CIRIAF – Interuniversity Research Center on Pollution and Environment, University of Perugia, Via G. Duranti, Perugia, Italy. E-mail address: [email protected] (E. Bonamente).

https://doi.org/10.1016/j.scitotenv.2017.09.254 0048-9697/© 2017 Elsevier B.V. All rights reserved.

Carbon and energy footprint studies provide significant clues about environmental impact of products and processes in order to enable

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decision makers to draw sustainable policies (Lueking and Cole, 2017). Tools such as the life cycle analysis (LCA (Manuilova et al., 2009)) are widely used to assess the impact of industrial activities on the environment and the feasibility of industrial processes, for project approval and implementation (Bonamente and Cotana, 2015; Rossi et al., 2016). In literature, several studies are available about the application of LCA to disparate processes in all the three sectors of economy (Bonamente and Cotana, 2015; Rossi et al., 2016; Rinaldi et al., 2014; Girgenti et al., 2013; Bessou et al., 2013; Bonamente et al., 2016a). The transition to the so-called circular economy sets urgent objectives to be addressed, such as the increase of renewable energy production and the reduction of wastes and CO2 emissions (Geissdoerfer et al., 2017; Castellani et al., 2014a). In this scenario, the environmental performance of energy-related products assumes a crucial role, which can be determined through the use of LCA tools (Abusoglu et al., 1684). The biogas production from organic wastes and its subsequent purification, is a renewable alternative for power generation and a promising route for maximizing waste valorization (Demirbas et al., 2011). In order to convert the biogas into biomethane, a purification section is necessary. The so-produced upgraded biogas can be used as a substitute for methane. The first upgrading stage is the CO2 removal which is initially in a volume fraction from 35% to 55%. Other contaminants, such as sulfur and siloxanes, have also to be removed. Several and well-established processes are commonly used for CO2 removal: physical absorption with water or organic solvents, amine scrubbing, pressure swing adsorption, membrane separation, cryogenic separation, and biological removal (Rotunno et al., 2017; Ryckebosh et al., 2011; Bonamente et al., 2016b). In addition to the above-cited processes, several studies have proved the benefits obtainable from clathrate hydrate separation (Castellani et al., 2014b; Xi et al., 2016; Sales Silva et al., 2016). Clathrate hydrates are non-stoichiometric crystalline compounds, formed by different types of guest molecules enclathrated in cages of water molecules at moderate conditions (low temperature and high pressure) (Sloan, 1998). Each hydrate-forming gas has its own hydrate formation conditions. On the basis of this principle, gas hydrates are applied to gas mixture separation: operating temperature and pressure can be chosen appropriately in order to have one component only in the hydrate phase (Castellani et al., 2016). It is well known that CO2 forms hydrates under higher temperature or lower pressure than CH4 (Sloan, 1998), so they can be selectively separated from the initial mixture, resulting in CH4 enriched biogas. The separated CO2 can be recovered dissociating the hydrate phase. Hydrate-based gas separation method presents some theoretical benefits, such as: i) moderate operating temperature with a temperature difference between formation and decomposition of 10 K, lower than that of other upgrading methods; ii) the separated gases are already compressed; iii) hydrates main component is water, together with not volatile additives, which can be recirculated with no material loss (Xi et al., 2016). In accordance with the definition of circular economy, as a regenerative system in which waste, emission, and energy leakage are minimised by closing material and energy loops (Geissdoerfer et al., 2017), the present paper proposes to exploit the CO2 separated from biogas via gas hydrate crystallization as a renewable carbon source for making additional CH4 through the Sabatier reaction (Xu and Moulijn, 1996). The integration of the hydrate-based biogas upgrading with the conversion of the recovered CO2 in synthetic methane contributes to the mitigation of global climate changes and provides an energy storage solution to compensate for the fluctuations in intermittent renewable energy production (Nastasi and Lo Basso, 2016; Lo Basso et al., 2017; Bonamente et al., 2015; Bonamente et al., 2016c). Renewable energy (solar radiation) is in fact used to produce hydrogen, which is exothermically combined with carbon dioxide in the Sabatier reaction to obtain methane. The process, formed by a section of hydrate-based biogas upgrading and a section of methanation of the CO2 previously separated, was studied on an experimental small size pilot plant. Experimental data were

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used to carry out an energy and carbon footprint analysis of the process. A detailed discussion of the outputs is presented together with a comparison with other results of similar processes. Several studies have been published in the past years on LCAs of H2 production, with a focus on the use of renewable energy (Bonamente et al., 2015; Bonamente et al., 2016c; Dufour et al., 2009; Patyk et al., 2013). There are also some environmental studies on several techniques of biogas upgrading (Morero et al., 2015; Xu et al., 2015). Nevertheless, only a few environmental studies have been conducted on CH4 production by power-to-gas: in particular, a life-cycle approach to evaluate GHG emissions (Reiter and Lindorfer, 2015), an environmental analysis on the biogas upgrading integrated with power-to-gas process (Collet et al., 2016), and renewable heat deliverable using smart solutions (Nastasi and Lo Basso, 2017). To our knowledge, this is the first time that an environmental assessment is performed to evaluate the production of bioCH4 from hydrate-based biogas upgrading coupled with power-to-gas technology. 2. Process description The process is formed by the biogas upgrading section and the CO2 methanation section. A schematic diagram of the process is shown in Fig. 1. The biogas upgrading reactor, already used and described in detail in previous works for methane and carbon dioxide hydrate formation processes (Castellani et al., 2014b; Brinchi et al., 2014), is a high-pressure cylindrical AISI 304 stainless steel vessel with a total internal volume of 0.025 m3 (25 L). The reactor is used to produce carbon dioxide hydrates from a biogas mixture through spraying aqueous solution into the gas phase. The aqueous solution consists of water and a promoter (sodium dodecyl sulphate, SDS). It is loaded in the reactor and the input gas (e.g. biogas) is bubbled into the aqueous solution through the 5 check valves situated in the lower manifold. The mixture is pressurized to the target experimental pressure and then cooled. Once the experimental conditions are reached, aqueous solution is recirculated by the pump and sprayed by the nozzles of the upper manifold into the gas phase. Each test is carried out with a constant internal pressure. At the end of each test, part of the CO2 is trapped inside hydrates and methane-enriched biogas is vented out from gas outlet port and collected. The solid hydrate phase is then dissociated (i.e. melted) to recover the separated carbon dioxide. The biogas upgrading process via hydrate formation was carried out at 4.5 MPa, starting from a biogas composition ad described in Table 1. The initial internal temperature was set to 275.1 K. The amount of water loaded at the beginning of each experiment is 5 kg, the concentration of the SDS promoter being 300 mg/kg. The differential pressure on nozzles was 1.0 MPa for all experimental tests. Recirculation time of the aqueous solution was 60 min. The analysis of the gas composition contained in the hydrates after dissociation shows that the volume fraction of CO2 is equal to 94.8%, while the volume fraction of CH4 is 5.2%. The final composition after the 60 minute cycle, is 70.1% CH4, 22.4% CO2, and lower concentration of N2, H2, and O2 respectively. The total upgraded biogas produced during the experimental test was 0.89 Nm3. The CO2 methanation apparatus consists of CO2-H2 mixing section, a heating section, a Sabatier reactor, and a water separation section (Fig. 1, right). It operates in continuous mode: the separated CO2 is sent to the mixing section where it is combined with hydrogen to reach the stoichiometric ratio (1:4), in accordance with the reaction in (1) CO2 þ 4 H2 →CH4 þ 2 H2 O

ð1Þ

Methane and water are produced in a moderately exothermic reaction (H° = −165 kJ/mol) (Brooks et al., 2007). The gaseous mixture is then flown, with a rate of 0.63 m3/h, through a pre-heater section

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Fig. 1. Schematic of the process.

(consisting of band heaters, provided by Watlow) before reaching the reactor. The Sabatier reactor is a cylindrical AISI 304 stainless monotubolar fixed bed reactor with a diameter of 1 in (25.4 mm) and a length of 300 mm. It is filled with Ni catalyst pellets. The reaction products pass through the water separation section, which consists of a 6 mm stainless steel cooling coil, immersed in a thermostatic bath. In this final section,

Table 1 Composition and energy content of input and output fuels. Fuel

Biogas

Methane-enriched biogas

Synthetic fuel

Biosynthetic methane

Hydrogen

Component

CH4 CO2 H2 O2 N2 CH4 CO2 H2 O2 N2 CH4 CO2 H2 CH4 CO2 H2 O2 N2 H2

water vapour condensation occurs and the gaseous incondensable products are collected. The experimental test for CO2 methanation was carried out at 2.0 MPa and at an internal temperature of 724 K, using a stoichiometric CO2/H2 ratio. The total CO2/H2 mixture sent to the reactor was 0.66 Nm3. CO2 methanation produced a gaseous mixture (i.e. synthetic methane, 97.2% CH4), as shown in Table 1, corresponding to a CO2 conversion efficiency of 94.9%. The total biosynthetic methane (upgraded biogas + synthetic methane) was 0.962 Nm3. 3. Life-cycle methodology

Volume fraction

HHVcomponent

HHVfuel

(%)

(kJ/mol)

(kJ/mol)

(MJ/Nm3)

55.00 37.50 0.50 2.00 5.00 70.10 22.40 0.50 2.30 4.70 97.24 1.02 1.74 73.82 19.47 0.67 1.98 4.06 100

891.5 0 286.2 0 0 891.5 0 286.2 0 0 891.5 0.0 286.2 891.5 0.0 286.2 0.0 0.0 286.2

491.76

20.86

626.38

26.56

871.88

36.95

660.03

27.98

286.2

12.1

The life-cycle carbon and energy footprint are evaluated with the SimaPro 8.2 software (PRé Consultants) in accordance with ISO/TS 14067 (ISO, 2013) and the ISO 14040 series (ISO, 2006a; ISO, 2006b). The standard carbon footprint (CF) methodology (IPCC 2013 GWP 100a) is used. The life-cycle primary energy consumption is also computed selecting the Cumulative Energy Demand assessment method. The cumulative energy demand of a product or system throughout the entire life cycle includes both the direct and indirect energy use, allowing the calculation of the energy footprint (EF). 3.1. The goal and scope definition The goal of this study is to evaluate the primary energy consumption and the GHG emissions associated to the production of biosynthetic methane via the coupled process of hydrate-based biogas upgrading and CO2 methanation. The reference functional unit of this LCA is the unitary physical amount of produced biosynthetic methane (expressed either in kg or in Nm3). As an additional functional unit, the unitary

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energy content (expressed in MJ) of produced biosynthetic methane was also considered to evaluate the energy efficiency of the process. The system boundaries include all the phases from production of input materials (reagents and water) and energywares (electricity, non-upgraded biogas, and hydrogen) to the assembly of the final product (biosynthetic methane) which is a mixture of methane-enriched biogas (from the upgrading section) and synthetic fuel (from the methanation section). The system boundaries are given in Fig. 2.

Table 2 Inventory data for the production of 0.962 Nm3 of biosynthetic methane. Phase

Input

Unit

Amount

Data source

Upgrading

Biogas Electricity Surfactant Water Hydrogen Electricity Catalyst

Nm3 kJ kg kg Nm3 kJ kg

0.89 821.5 0.0015 5.0 0.53 1135.9 1.04E-05

Measured data Measured data Measured data Measured data Measured data Measured data Measured data

Methanation

3.2. Life cycle inventory (LCI) The overall biosynthetic methane production during experimental activities was 0.962 Nm3, corresponding to 0.943 kg. It is obtained mixing 0.830 Nm3 (0.840 kg) of methane-enriched biogas and 0.132 Nm3 (0.103 kg) of synthetic fuel. The composition of gaseous fuels and their energy content (higher calorific value, HHV) is shown in Table 1. The evaluation of the energy consumption of the process was performed based on the results of the experimental campaign. The hydrate-based process energy costs are mainly related to biogas compression, water cooling and recirculation, and reaction heat removal. Biogas is compressed to 4.5 MPa starting from atmospheric pressure, with an initial temperature of 308 K (35 °C), through a 3-stage compression, and initially cooled to 293 K (20 °C) using the environment as heat sink. The complete cooling to 275 K (2 °C) is carried out using a chiller, with a coefficient of performance (COP) equal to 3. The water solution, loaded into the reactor, is cooled to 275 K via a cooling process with a COP equal to 3 and recirculated with a pump. Hydrate formation is an exothermic process, with an enthalpy of formation for CO2 of 73 kJ/mol (Castellani et al., 2014b). Reaction heat must be removed cooling the system. The CO2 release from hydrates is obtained via hydrate dissociation, simply providing enthalpy of dissociation with the use of heat from the environment. Hydrate dissociation is not an energy-consuming process. Site-specific data are used for the life cycle inventory as shown in Table 2. Energy costs related to the methanation process arise from hydrogen and carbon dioxide compression for injection in the synthesis reactor, energy consumption for reactor heating up to the synthesis temperature, and removal of reaction heat. Since CO2 is released from the hydrates at 4.5 MPa, only hydrogen, from the electrolyzer, is compressed to 2.0 MPa from atmospheric pressure, starting from an initial temperature of 293 K (20 °C), through a 3-stage compression. Hydrogen is then heated to 724 K from the final compression temperature of

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408 K, together with CO2, which is heated from 293 K to 724 K, in order to reach the operating conditions. Sabatier reaction is an exothermic process, with an enthalpy of formation of 165 kJ/mol. Reaction heat is removed through a cooling process. Water-methane separation is not energy-consuming since it occurs at environmental temperature. 4. Results Two sets of results are obtained for the biosynthetic methane production process. The first (i.e. laboratory setup) refers to the process as it was performed in the laboratory pilot plant: in this case, the required electricity was supplied by the grid. The network view of CF results is shown in Fig. 3. It can be noted that most of the impact (approx. 70%) comes from grid-energy consumption. Details of both CF and EF analyses for the laboratory setup case are given in Table 3. The total CF of the process is 1.875 kgCO2eq/Nm3, the synthetic biofuel phase being responsible for 65% of the impact because of the high energy consumption required by the electrolysis. The total EF is 40.57 MJ/Nm3 and the production of synthetic fuel is again the most impacting phase (approx. 55%). A more realistic estimate of the process footprint is performed considering that the electric energy, consumed within the process, is produced by photovoltaics (i.e. improved setup). In this case, the total CF and EF are 0.7081 kgCO2eq/Nm3 (approx. − 60%) and 28.55 MJ/Nm3 (approx. −30%), respectively. Such values can be considered as representative of the process capabilities, in terms of environmental sustainability, considering that integration of photovoltaic panels could be done straightforwardly for real applications. The network view of CF results is shown in Fig. 4. Details are shown in Table 4. In terms of energy efficiency, the ratio between the energy content of a biosynthetic gas to the primary energy needed for its production is 0.578 for the laboratory setup and 0.821 for the improved setup.

Fig. 2. System boundaries.

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Fig. 3. Network view of carbon footprint results for the laboratory setup.

4.1. Sensitivity analysis and comparison with other results Being hydrogen production one of the most impacting phases, because of the high energy consumption, a systematic analysis was performed (Table 5). Different cases of H2 production via electrolysis in both laboratory (I) and improved (II) setups are considered. For comparison, CF and EF is also shown using literature data for H2 taken from the market. In particular, two scenarios are given considering the average H2 production (III) and H2 production from electrolysis (IV) using data from the EcoInvent 3.2 database (Ecoinvent Version 3.2, n.d.). A drastic reduction is observed when the required energy is produced by photovoltaic panels (− 87% CF, − 90% EFnon-ren), which is very similar to that obtainable using the average market scenario (−89% CF, −75% EFnon-ren). As a result, the laboratory setup is very sensitive to the H2 production since approx. 50% of the impact is associated

with this process. Using H2 from the market would produce an overall decrease of both CF (− 51%) and EF (− 39%). The improved setup, on the other hand, already features an optimized process for H2 production and it is not sensitive to this parameter. The comparison with other results from literature for H2 production cannot be performed with the same detail because of the heterogeneity of assumptions and functional units used. However, a good consistency with other results is observed: 0.1–0.3 kgCO2eq/Nm3 considering advanced technologies and carbon capture and sequestration (Dufour et al., 2009); 0.1 kgCO2eq/MJ and 1.7 MJ/MJ using high-temperature electrolysis (Patyk et al., 2013); and 0.025–0.230 kgCO2eq/MJ computed including power from PV and grid (Reiter and Lindorfer, 2015). The comparison with other processes for biogas upgrading and CO2 valorization can only be performed partially, due to the lack of similar approaches. As a reference, the energy efficiency of biogas upgrading

Table 3 Carbon and energy footprint of the laboratory setup. Phase

Carbon footprint 3

Methane-enriched biogas Water Surfactant Electricity from grid Biogas Synthetic fuel Nickel oxide Hydrogen, electrolysis Electricity from grid Biosynthetic methane

Energy footprint

(kgCO2eq/Nm )

(kgCO2eq/MJ)

(%)

(MJ/Nm3)

(MJ/MJ)

(%)

0.6557 0.001929 0.004997 0.1211 0.5277 1.220 0.00006785 1.052 0.1675 1.875

0.02344 0.00006895 0.0001786 0.004329 0.01886 0.04359 0.000002425 0.03760 0.005986 0.06702

35.0% 0.10% 0.27% 6.46% 28.1% 65.0% 0.00% 56.1% 8.93% 100%

17.88 0.03711 0.07202 2.254 15.52 22.69 0.0009291 19.57 3.116 40.57

0.6391 0.001326 0.002574 0.0805 0.5546 0.8110 0.00003320 0.6996 0.1114 1.450

44.1% 0.09% 0.18% 5.55% 38.2% 55.9% 0.00% 48.2% 7.68% 100%

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409

Fig. 4. Network view of carbon footprint results for the improved setup.

only using pressured water scrubbing, monoethanolamine aqueous scrubbing and ionic liquid scrubbing is approximately 0.9 MJ/MJ (Xu et al., 2015) (it is 0.81 for the improved setup presented in this paper). A range between 0.01 and 0.3 kgCO2eq/MJ is estimated for CH4 production from power-to-gas including energy from PV and from the grid (Reiter and Lindorfer, 2015). A similar approach to the one presented in this paper is discussed in (Collet et al., 2016). In this case, different configurations are considered for heat production from natural gas, which is obtained using the electrolysis, methanation, and upgrading chain. The overall results (0.07–0.16 kgCO2eq/MJ) are consistent with the ones presented in this study (0.067 and 0.025 kgCO2eq/MJ for the laboratory and the improved setup, respectively). Finally, a direct comparison of CF and EF with literature values is performed using data from (Ecoinvent Version 3.2, n.d.). For 96%-CH4 biomethane CF is 0.5974 kgCO2eq/Nm3 and EF is 38.86 MJ/Nm3, corresponding to − 23% and + 36%, respectively. In case of 45%-CH4 biogas, CF and EF are 0.5699 kgCO2eq/Nm3 (− 27%) and 16.77 MJ/Nm3 (− 59%), respectively.

5. Discussion The LCA methodology was applied in a cradle-to-gate approach, including all the input material and production phases of a ready-to-use gas. Considering 1 Nm3 as functional unit of the study, CF and EF are 0.7081 kgCO2eq/Nm3 and 28.55 MJ/Nm3, respectively. The overall environmental footprint of the process is consistent with literature data for biomethane (96%-CH4), showing a slightly larger impact in terms of GHG emissions and a reduced impact in terms of primary energy requirements. In the laboratory setup, the CF and EF associated to the electricity from the grid supplied to the hydrate-based biogas upgrading process are respectively 6.46% and 5.55% (Table 3). Energy-consuming steps during the hydrate-based process are related to biogas compression, water cooling and recirculation, and reaction-heat removal. The contribution of the electricity supplied to the methane-enriched biogas process on the overall CF and EF is even lower if the consumed electric energy is from renewable sources (2.21% and 4.09% in Table 4).

Table 4 Carbon and energy footprint of the improved setup. Phase

Carbon footprint 3

Methane-enriched biogas Water Surfactant Electricity from grid Biogas Synthetic fuel Nickel oxide Hydrogen, electrolysis Electricity from grid Biosynthetic methane

Energy footprint

(kgCO2eq/Nm )

(kgCO2eq/MJ)

(%)

(MJ/Nm3)

(MJ/MJ)

(%)

0.5503 0.001929 0.004997 0.01566 0.5277 0.1578 0.00006785 0.1361 0.02166 0.7081

0.01967 0.00006895 0.0001786 0.0005598 0.01886 0.005641 0.000002425 0.004865 0.0007740 0.02531

77.7% 0.27% 0.71% 2.21% 74.5% 22.3% 0.01% 19.2% 3.06% 100%

16.80 0.03711 0.07202 1.167 15.52 11.75 0.0009291 10.137 1.614 28.55

0.6002 0.001326 0.002574 0.04171 0.5546 0.4200 0.00003320 0.3623 0.05767 1.020

58.8% 0.13% 0.25% 4.09% 54.4% 41.2% 0.00% 35.5% 5.65% 100%

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Table 5 Carbon and energy footprint of hydrogen production. Process

Hydrogen (I) Water Electricity from grid Hydrogen (II) Water Electricity from PV Hydrogen (III) Hydrogen (IV)

Carbon footprint

Energy footprint

(kgCO2eq/Nm3)

(kgCO2eq/MJ)

(MJ/Nm3)

(MJ/MJ)

(MJnon-ren./MJ)

1.910 0.00014007 1.910 0.2471 0.00014007 0.2469 0.20512 1.175

0.1578 0.00001157 0.1578 0.02041 0.00001157 0.02040 0.01695 0.09705

35.53 0.002694 35.53 18.40 0.002694 18.40 7.469 24.643

2.935 0.0002226 2.935 1.520 0.0002226 1.520 0.6171 2.036

2.474 0.0001990 2.474 0.2389 0.0001355 0.2388 0.6069 1.789

(I) Laboratory setup: hydrogen is produced via electrolysis and electricity is provided by the grid. (II) Improved setup: hydrogen is produced via electrolysis and electricity is provided by photovoltaic panels. (III) Average market scenario. (IV) Alternative market scenario: hydrogen is produced from electrolysis.

The methane volume fraction in the methane-enriched biogas (after biogas upgrading) is 70.10%. It reaches 73.82% in the biosynthetic methane (Table 1) thanks to the mixing with synthetic methane from CO2 valorization. The energy content is equal to 22.24 MJ/Nm3 for the methane-enriched biogas and 23.44 MJ/Nm3 for the biosynthetic methane. The obtained values, however, do not address the quality standards for transportation or injection into the grid. For example, Swedish national standard for biomethane demands a total CO2/O2/N2 volume fraction lower than 5%; Swiss national standard for unlimited gas injection requires a methane content higher than 96%, while French national regulation for gas injection foresees a lower heating value from 34.2 to 37.8 MJ/Nm3 (Persson et al., 2006). Considering that the energy consumption for the hydrate-based separation process has a very low impact on the overall CF and EF, a multistage hydrate production process could be performed in order to reach higher methane content and heating values in order to comply with the biomethane quality standards for grid injection. As far as the methanation process is concerned, energy-consuming phases consist in hydrogen and carbon dioxide compression for injection in the synthesis reactor, energy consumption for reactor heating up to the synthesis temperature, and removal of reaction heat. The CF and EF associated to the electricity from the grid supplied to the CO2 methanation process are respectively 8.93% and 7.68% (Table 3). With the use of renewable energy from photovoltaic systems, such values are 3.06% and 5.65%, respectively (Table 4). The contribution of the electricity from the grid is negligible if compared to hydrogen electrolysis, which has an impact of 56.1% on the overall CF and of 48.2% on the EF of the integrated process. If the electrolysis is carried out using renewable photovoltaic energy, its CF drops from 1.052 kgCO2eq/Nm3 to 0.1361 kgCO2eq/Nm3, thus affecting the overall CF and EF to a lower extent: 19.2% and 35.5%, respectively. In addition to the potentialities for environmental footprint reduction, producing hydrogen using renewable solar energy can be considered an important technological option for renewable energy storage. Electrolysis has the potential to transform sustainable electrical energy into green hydrogen. In our process, it is subsequently converted to synthetic methane via methanation, with the feed-in of hydrate-based separated CO2, which is again of biological origin. The production of biosynthetic methane, in a context of increasing implementation of renewable energy, may help to overcome crucial renewable source related challenges, such as fluctuating electricity production and increasing the balancing needed in temporal and spatial domains (Götz et al., 2016). Solar electricity production is in fact weather dependent and therefore it needs to be in balance with the electricity demand. Possible solutions involve demand-side management, additional flexible power generation, as well as energy storage systems. Our process can be considered as a chemical storage option, in which solar renewable electricity is stored in biosynthetic methane. As a sustainable and versatile energy carrier, it can be used for reconversion to

electricity, for heating and cooling purposes, and as an alternative fuel option for the transport sector and the natural gas distribution infrastructure, which offers the largest storage capacity for energy. When the production process makes use of renewable energy sources to cover the electric energy demand (e.g. via photovoltaic panels), the overall energy efficiency of the proposed process exceeds 0.82, meaning that only 18% of the required primary energy is not converted into usable and easily storable energy (biosynthetic methane). The worldwide average energy efficiency for fossil methane being 0.75. 6. Conclusion Carbon and energy footprint of a two-stage process for the production of biosynthetic methane, a mix of renewable fuels obtained from biogas upgrading and methanation of residual biological CO2, is presented. The innovative process separates a large portion of CO2 from the input biogas in a hydrate-based biogas upgrading process, thus resulting in a methane-enriched biogas, and converts the separated CO2 in additional synthetic methane via Sabatier reaction. The objective of the paper is to evaluate the GHG emissions and the primary energy consumption associated to the production of biosynthetic methane through a cradle-to-gate LCA approach. The functional unit is the unitary amount of produced biosynthetic methane. Impacts are also computed in terms of unitary energy content. Carbon and energy footprints are 0.7081 kgCO2eq/Nm3 and 28.55 MJ/Nm3, respectively, when the electric energy required by the process is provided by photovoltaic panels. In this scenario, the overall energy efficiency is about 0.82, higher than the worldwide average energy efficiency for fossil methane, which is 0.75. Since hydrogen production is one of the most impacting phases, a sensitivity analysis was performed. Results show that there is a drastic reduction of the environmental footprint when the required energy is produced by photovoltaic panels. In addition, hydrogen production from renewable solar energy is a viable option to overcome issued related to the non-programmability of renewable resources via the storage of solar energy in usable fuels. Acknowledgement The research presented in this paper is a part of the project “BIT3G”, which is funded by the Italian Ministry of Scientific Research and University (grant no. CTN01_00063). The authors would like to acknowledge also Novamont for its efforts in coordinating the consortium. References Abusoglu, A., Ozahi, E., Kutlar, A.I., Al-jaf, H., 1684-1692. Life cycle assessment (LCA) of digested sewage sludge incineration for heat and power production. J. Clean. Prod. 2017, 142.

B. Castellani et al. / Science of the Total Environment 615 (2018) 404–411 Bessou, C., Basset-Mens, C., Tran, T., Benoist, A., 2013. LCA applied to perennial cropping systems: a review focused on the farm stage. Int. J. Life Cycle Assess. 18, 340–361. Bonamente, E., Cotana, F., 2015. Carbon and energy footprint of prefabricated industrial buildings: a systematic life cycle assessment analysis. Energies 8, 12685–12701. Bonamente, E., Pelliccia, L., Merico, M.C., Rinaldi, S., Petrozzi, A., 2015. The multifunctional environmental energy tower: carbon footprint and land use analysis of an integrated renewable energy plant. Sustainability 7, 13564–13584. Bonamente, E., Scrucca, F., Rinaldi, S., Merico, M.C., Asdrubali, F., Lamastra, L., 2016a. Environmental impact of an Italian wine bottle: carbon and water footprints assessment. Sci. Total Environ. 560–561, 274–283. Bonamente, E., Aquino, A., Nicolini, A., Cotana, F., 2016b. Experimental analysis and process modeling of carbon dioxide removal using tuff. Sustainability 8 (12), 1258. Bonamente, E., Moretti, E., Buratti, C., Cotana, F., 2016c. Design and monitoring of an innovative geothermal system including an underground heat-storage tank. International Journal of Green Energy 13 (8), 822–830. Brinchi, L., Castellani, B., Rossi, F., Cotana, F., Morini, E., Nicolini, A., Filipponi, M., 2014. Experimental investigations on scaled-up methane hydrate production with surfactant promotion: energy considerations. J. Pet. Sci. Eng. 120, 187–193. Brooks, K., Hu, J., Zhu, H., Kee, R., 2007. Methanation of carbon dioxide by hydrogen reduction using the Sabatier process in microchannel reactors. Chem. Eng. Sci. 62, 1161–1170. Castellani, B., Morini, E., Filipponi, M., Nicolini, A., Palombo, M., Cotana, F., Rossi, F., 2014a. Comparative analysis of monitoring devices for particulate content in exhaust gases. Sustainability (Switzerland) 6 (7), 4287–4307. Castellani, B., Rossi, F., Filipponi, M., Nicolini, A., 2014b. Hydrate-based removal of carbon dioxide and hydrogen sulphide from biogas mixtures: experimental investigation and energy evaluations. Biomass Bioenergy 70, 330–338. Castellani, B., Rossetti, G., Tupsakhare, S., Rossi, F., Nicolini, A., Castaldi, M.J., 2016. Simulation of CO2 storage and methane gas production from gas hydrates in a large scale laboratory reactor. J. Pet. Sci. Eng. 147, 515–527. Collet, P., Flottes, E., Favre, A., Raynal, L., Pierre, H., Capela, S., Peregrina, C., 2016. Technoeconomic and life cycle assessment of methane production via biogas upgrading and power to gas technology. Appl. Energy https://doi.org/10.1016/j.apenergy.2016.08.181 (In Press, Corrected Proof). Demirbas, M.F., Balat, M., Balat, H., 2011. Biowastes-to-biofuels. Energy Convers. Manag. 52, 1815–1828. Dufour, J., et al., 2009. Life cycle assessment of processes for hydrogen production. Environmental feasibility and reduction of greenhouse gases emissions. Int. J. Hydrog. Energy 34, 1370–1376. Ecoinvent Version 3.2, d. Available online:. http://www.ecoinvent.org/database/ ecoinvent-32/ecoinvent-32.html. Geissdoerfer, M., Savaget, P., Bocken, N.M.P., Hultink, E.J., 2017. The circular economy - a new sustainability paradigm? J. Clean. Prod. 143, 757–768. Girgenti, V., Peano, C., Bounous, M., Baudino, C., 2013. A life cycle assessment of nonrenewable energy use and greenhouse gas emissions associated with blueberry and raspberry production in northern Italy. Sci. Total Environ. 458–60, 414–418. Götz, M., Lefebvre, J., Mörs, F., McDaniel Koch, A., Graf, F., Bajohr, S., Reimert, R., Kolb, T., 2016. Renewable power-to-gas: a technological and economic review. Renew. Energy 85, 1371–1390. ISO (International Standard Organization), 2006a. ISO 14044:2006a Environmental Management—life Cycle Assessment—requirements and Guidelines. International Organization for Standardization (ISO), Geneva, Switzerland.

411

ISO (International Standard Organization), 2006b. ISO 14040:2006b Environmental Management—life Cycle Assessment—principles and Framework. International Organization for Standardization (ISO), Geneva, Switzerland. ISO (International Standard Organization), 2013. ISO/TS 14067:2013 Greenhouse Gases—carbon Footprint of Products—requirements and Guidelines for Quantification and Communication. International Organization for Standardization (ISO), Geneva, Switzerland. Lo Basso, G., Nastasi, B., Astiaso Garcia, D., Cumo, F., 2017. How to handle the hydrogen enriched natural gas blends in combustion efficiency measurement procedure of conventional and condensing boilers. Energy 123, 615–636. Lueking, A.D., Cole, M.W., 2017. Energy and mass balances related to climate change and remediation. Sci. Total Environ. 590–591, 416–429. Manuilova, A., Suebsiri, J., Wilson, M., 2009. Should life cycle assessment be part of the environmental impact assessment? Case study: EIA of CO2 capture and storage in Canada. Energy Procedia 1, 451–4518. Morero, B., Rodriguez, M.B., Campanella, E.A., 2015. Environmental impact assessment as a complement of life cycle assessment. Case study: upgrading of biogas. Bioresour. Technol. 190, 402–407. Nastasi, B., Lo Basso, G., 2016. Hydrogen to link heat and electricity in the transition towards future Smart Energy Systems. Energy 110, 5–22. Nastasi, B., Lo Basso, G., 2017. Power-to-gas integration in the transition towards future Urban Energy Systems. Int. J. Hydrog. Energy 42 (38), 23933–23951. Patyk, A., Bachmann, T.M., Brisse, A., 2013. Life cycle assessment of H2 generation with high temperature electrolysis. Int. J. Hydrog. Energy 38, 3865–3880. Persson, M., Jönsson, O., Wellinger, A., December 2006. Biogas Upgrading to Vehicle Fuel Standards and Grid Injection. IEA Bioenergy, Sweden. Reiter, G., Lindorfer, J., 2015. Global warming potential of hydrogen and methane production from renewable electricity via power-to-gas technology. Int. J. Life Cycle Assess. 20, 477–489. Rinaldi, S., Barbanera, M., Lascaro, E., 2014. Assessment of carbon footprint and energy performance of the extra virgin olive oil chain in Umbria, Italy. Sci. Total Environ. 482–483, 71–79. Rossi, F., Bonamente, E., Nicolini, A., Anderini, E., Cotana, F., 2016. A carbon footprint and energy consumption assessment methodology for UHI-affected lighting systems in built areas. Energy Build. 114, 96–103. Rotunno, P., Lanzini, A., Leone, P., 2017. Energy and economic analysis of a water scrubbing based biogas upgrading process for biomethane injection into the gas grid or use as transportation fuel. Renew. Energy 102, 417–432. Ryckebosh, E., Drouillon, M., Vervaeren, H., 2011. Techniques for transformation of biogas to biomethane. Biomass Bioenergy 35, 1633–1645. Sales Silva, L.P., Dalmazzone, D., Stambouli, M., Arpentinier, P., Trueba, A., Fürst, W., 2016. Phase behavior of simple tributylphosphine oxide (TBPO) and mixed gas (CO2, CH4 and CO2 + CH4) + TBPO semiclathrate hydrates. J. Chem. Thermodyn. 102, 293–302. Sloan, E.D., 1998. Clathrate Hydrates of Natural Gases. Marcel Dekker, New York. Xi, Z.M., Li, X.S., Chen, Z.Y., Li, G., Yan, K.F., Xu, C.G., Cai, J., 2016. Hydrate-based CO2 capture and CH4 purification from simulated biogas with synergic additives based on gas solvent. Appl. Energy 162, 1153–1159. Xu, X.D., Moulijn, J.A., 1996. Mitigation of CO2 by chemical conversion: plausible chemical reactions and promising products. Energy Fuel 10, 305–325. Xu, Yajing, Huang, Ying, Wu, Bin, Zhang, Xiangping, Zhang, Suojiang, 2015. Biogas upgrading technologies: energetic analysis and environmental impact assessment. Chin. J. Chem. Eng. 23, 247–254.