Appendix – Life cycle inventory

SOFC system manufacturing and maintenance

The SOFC system is subdivided into three main parts:

- SOFC stack;- Electrical BoP (inverters, cables, etc.);- Mechanical BoP (heat exchangers, piping, support structure, etc.).

All the information about stack manufacturing is described in first sub-chapter, while the base model for BoP manufacturing is taken from Ecoinvent® database and explained in the second section.

SOFC stack manufacturing

From Ecoinvent® database inventories are available for a 125 kWe SOFC fuel cell system, tubular configuration, for stationary operation as CHP [1]. In particular, unit processes are available for: stack manufacturing, BOP manufacturing and system assembly, SOFC operation and SOFC maintenance. Given the resemblance to the case study, these inventories are taken as base model, to be modified and updated in order to match the requirements of this study. The two main reasons for the need of modifying database are different architecture for SOFC technology, which leads to different use of materials and possibly different manufacturing processes, and the fact that information come from a study published in the early 2000s [2]. In more than 10 years many changes and improvements in the manufacturing process have been achieved. Information about cell manufacturing was provided by Istituto di Scienza e Tecnologia dei Materiali (ISTEC), a division of Consiglio Nazionale delle Ricerche (CNR). Data are referred to the production of a planar rectangular cell with 81 cm2 active area.

In Table are presented material composition for cell layers, thickness of each layer and manufacturing procedures, related to ISTEC information.

Table A1. SOFC manufacturing inventory.

Manufacturing procedureCell element Material

compositionName ThicknessTape casting Anode (support) 450 µm NiO-YSZ1 70/30 WtScreen printing Anode (functional) 20 µm NiO-YSZ 70/30 WtScreen printing Electrolyte 10 µm YSZCo-firing of anode and electrolyte at 1400°C for 4 hours

Screen printing Cathode 30 µm LSM2-YSZ 50/50 Wt

1 YSZ: Yttrium-stabilized Zirconium (Yttrium 8%mol)

Sintering at 1200°C for 2 hours

To scale these values to the requested size (250 kWe), cell voltage and cell current density need to be defined. Cell voltage is assumed as 0.8 V and current density is taken as 0.5 A/cm2 (assumptions). Also, according to stack developers, SOFC stack lifetime is taken as 6 years, instead of the 3-4 years of actual operation conditions.

Other than materials for cell production, information about emissions during manufacturing phase and energy consumptions were also provided. Emissions are related to the preparation of the slurries for tape casting and screen printing of cell layers and to the evaporation of solvents in co-firing and sintering phases. Energy consumption is entirely electricity, since only electric ovens are employed.

Missing materials in Ecoinvent® database are replaced with others according to their function or chemical family. Excluded materials are mainly solvents and plasticizers (<1% of total mass). Inventories for materials as YSZ and LSM are not available at Ecoinvent®

database and are retrieved from [3].

Energy consumption, even if available from the ISTEC lab, is sensible to scaling factor and is thus taken from the literature, referring to more industrialized processes [3]. Since the objective pursued modeling SOFC system manufacturing is to remain as general as possible, for electricity consumption, the European production mix is used.

Materials required for interconnect (electric connection between cells), cell support, piping and casing are taken from a merged inventory available from [3].

Table A2. SOFC manufacturing inventory.

Materials/energy flows Value Unit

NiO 150 kg8YSZ 67 kgLSM 0.62 kgEthanol (solvent) 28 kgMethyl ethyl ketone (solvent) 54 kgBenzyl alcohol (solvent) 8.26 kgCarbon black 0.46 kgModified starch (binder) 10.6 kgEthylene glycol (binder) 9.03 kgStainless Steel (Cr alloy) interconnecta 2’500 kgStainless Steel (Cr alloy) casingb 1’000 kgElectricity, medium voltage, UCTEc 55 MWhEmissions to air

Carbon dioxide 108 kgEthanold 28 kgMethyl ethyl ketoned 54.24 kgBenzyl alcohold 8.26 Kg

2 LSM: Lanthanum-Strontium-Manganite.

2

a) Taken as 10 kg/kW [3].b) Taken as 4 kg/kW [3].c) Assumed as 0.0885 kWh/cm2 of active area [3].d) 100% of solvent evaporates.

SOFC BoP manufacturing and system assembly inventory

As for stack inventory, Ecoinvent® database reports data for a 125 kWe tubular SOFC power unit, and even in this case the main source is the same used for stack inventory [2]. Nevertheless, the inventory proposed by Ecoinvent® contain a number of modifications to the original data, some of which will be adapted to the case study, too [1].

Starting from electrical BoP, the most important element is the inverter. Ecoinvent®

provides an inventory for a 2500 W inverter. Other electronic components (e.g. controllers) and electric components (e.g. cables) are not included in the inventory.

Mechanical BOP, on the other hand, includes all the materials related to components for the cleaning system, reformer, heat exchangers, piping, casing and structural support. Weight information is retrieved at Bloom Energy (www.bloomenergy.com). From the total weight, the amount of steel is determined subtracting the weight of all other known components (stack, inverters, sorbent materials, etc.).

Being fed with biogas, there is need for a cleaning component, for impurities removal. This feature is needed because the impurities contained in the biogas (mainly hydrogen sulfite (H2S), siloxanes3 and volatile organic compounds (VOCs) [4]) may damage the fuel cell, reducing drastically its life.

Figure A1. Scheme of impurities removal from biogas [4].

In Figure is presented the scheme for impurities removal presented in [4]. Materials used in the cleaning units are iron oxide for H2S removal and activated carbons for siloxanes and VOCs removal. The present study follows this scheme for the cleaning system, except

3 Siloxanes: organic silicon compounds typically used in such products as personal hygiene products; when present in the fuel gas, these compounds can lead to the formation and deposition of SiO 2 that can affect many components of the fuel cell system, such as heat exchangers, catalysts, and sensors.

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for the high temperature polisher, which is not included. The quantities of iron oxide and activated carbons for the 250 kWe SOFC are calculated as kg/kW of catalyst material. Since in Ecoinvent® there is no inventory for activated carbons, this material is substituted with a zinc oxide based catalyst, since it is able to fulfill the same cleaning function [5].

The reformer, necessary to transform the CH4 in CO and H2, is realized through the use of a catalyst, which therefore needs to be accounted in the inventory. The catalyst chosen for the case study is composed at 63% of alumina, 20% Nickel and the rest is taken as silicon [6]. A new process, including only the materials needed for the production of such component, is set. The quantity of catalyst needed is calculated as 2 m3, according to [7], which is then converted to mass using the bulk density of the material, taken as 970 kg/m 3

[6].

In order to take account of reformer, heat exchangers and part of the piping which work at high temperature, part of the steel for BoP is considered as stainless steel. Since no specific information is available, the same quantity, expressed in kg/kW, reported in [1], is used.

To model system assembly, metal forming processes and energy consumptions are used. As well as for stack inventory, electricity is taken as European production mix and the same applies for natural gas.

No direct emissions are reported during BoP manufacturing and system assembly.

SOFC operation inventory

Inventories for SOFC operation are presented both for natural gas and biogas operation mode. Ecoinvent® SOFC inventories are used as base models, modified accordingly to match the goal of the present study.

No specific information is required for the fuel chain of natural gas, since it is already available at Ecoinvent® database (natural gas from Italian national network is used).

On the other hand, biogas composition and LHV have to be specified. Since the modeled product system refers to SMAT facilities for biogas production, biogas characterization is done accordingly, following the information presented in [8].

Fundamental operation parameters are electrical and thermal efficiencies. In the present work, these values are chosen according to [8], where the simulations carried reported a 52% electrical efficiency and 33% thermal efficiency. Data are confirmed by SOFC producers as Convion (www.convion.fi) which declares an electrical efficiency of 53%.

Based on personal communication from stack developers, the average capacity factor is assumed as 90%. In Table the energy performance of the SOFC is shown, both for biogas and natural gas operation.

Table A3. SOFC electricity and heat production and fuel consumption.

Description Value UnitElectrical efficiency 52%

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Thermal efficiency 33%Average capacity factor 90%Electrical capacity 250 kWThermal capacity 159 kWFuel consumption 481 kWElectricity production 1’970 MWh/yHeat production 1’250 MWh/yBiogas consumption 634’700 Nm3/yNatural gas consumption 380’300 Nm3/y

Concerning the SOFC emissions modeling, since no direct measurements are available, a model is developed using technical datasheets from FC producers. For CO2, emissions are calculated as the stoichiometric amount needed to perform the oxidation of CH4. For biogas, to this amount is then summed the CO2 already present in the fuel. Other emissions, such as carbon monoxide (CO), nitrogen oxides (NOx), volatile organic compounds (VOCs), sulfur oxides (SOx), are taken from SOFC manufacturers datasheets (www.bloomenergy.com). For natural gas operation, these emissions are used as reported in the datasheets, for biogas operation the same values are scaled on CH4 content.

As introduced before, SOFC operation presents a case of multi-functionality, producing at the same time electricity and heat. The procedure chosen to solve this issue is allocation based on the exergetic content of the energy flows.

In order to evaluate the allocation factors for electricity and heat, the temperature at which the heat is recovered is needed. This temperature is taken as 90°C, the temperature of the water for anaerobic digester heating, in outlet from the recovery heat exchanger at the SOFC system.

Then, the energy flows must be calculated, on the same basis, to maintain the coherence of the inventory. The reference value for operation inventory is chosen as the Nm3 of consumed fuel. At last, the exergy content of the energy fuels is calculated. For electricity, the conversion factor is 1, while for heat the conversion is performed using Carnot coefficient, defined as:

Φ=1−T a

T 0

Where:

T a is the ambient temperature; T 0 is the temperature at which the heat is recovered.

In Table and A5 the results of allocation factors calculation are reported. All inputs and outputs of operation inventories are divided between electricity and heat according to these factors.

Table A4. Calculation of allocation factors for SOFC operation.

Description Value Unit

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Natural gas BiogasHeat temperature 90 90 °CAmbient temperature 20 20 °CElectricity exergy coefficient 1 1 Heat exergy coefficient 0.193 0.193 Electricity exergy flow 5.183 3.106 kWh/Nm3

Heat exergy flow 0.634 0.380 kWh/Nm3

Total exergy flow 5.817 3.486 kWh/Nm3

Electricity allocation factor 89.10% 89.10% Heat allocation factor 10.90% 10.90%

Table A5. Inventory for BOP manufacturing and system assembly (for 1 unit, stack + BoP).

Materials/energy flows Value Unit

SOFC stack, 250 kWe

Reinforcing steel 12’200 kgStainless steel (Cr alloy) 3’600 kgSheet rolling, steel (metal forming) 12’200 kgSheet rolling, stainless steel 3’600 kgReformer catalyst 1’970 kgZinc oxide (cleaning unit) 4’080 kgIron oxide (cleaning unit) 3’050 kgInverter 100 pNatural gas, burned in industrial furnace 122 GJElectricity, medium voltage, production UCTE 12 MWh

SOFC maintenance inventory

The first and most important maintenance element regards stack replacement. SOFC stack lifetime is taken as 6 years. To take account of this in the inventory, a substitution of 1/6 of stack is included in each yearly intervention.

Other important maintenance aspects are related to the substitution of the cleaning unit adsorbent materials and the reformer catalyst. Information about the materials for the cleaning unit is taken from [4], scaled to the power of the case study. For reformer catalyst, the amount to be substituted in each intervention is computed taking account that the entire charge of catalyst is replaced every 4 years.

Other maintenance requirements (e.g. malfunctioning parts, occasional damage) are modeled as the substitution of an amount of steel, accounted as a percentage of the steel composing the system. The percentages are chosen as 1% of BoP’s reinforcing steel and 1% of BoP’s stainless steel.

In maintenance inventory, deionized water consumption for start-ups is also considered. The amount of water can be calculated starting from the steam to carbon ratio (S/C). The SOFC under study uses an S/C of 2. According to [9], this ratio can be expressed as:

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S/C=nH2 O

nCH4

Where:

nH 2 O molar flow rate of steam; nCH 4

molar flow rate of methane;

Then, using 24 hours as a reasonable time to bring the system to full operation, and hypnotizing 2 stops per year, it is possible to calculate the inventory voice for deionized water in respect to each maintenance intervention.

Table reports the inventory for SOFC maintenance.

Table A6. Inventory for SOFC maintenance (for 1 complete yearly maintenance).

Materials/energy flows Value UnitStack replacementReinforcing steel 122 kgChromium steel 18/8 36 kgDeionized Water 3’700 kgReformer catalyst 492 kgIron oxide (cleaning unit) 6’100 kgZinc oxide (cleaning unit) 1’286 kg

Fuel chains: natural gas and wastewater biogas

This section provides the inventories for the two scenarios for the fuel chain, natural gas and wastewater biogas.

Natural gas

In this scenario, the assumption is to take the fuel from the national distribution network. Inventories for the distribution of natural gas at national level are already included in Ecoinvent® database and the fuel is considered as background data.

Wastewater biogas

The construction of inventories for the manufacturing and operation of the anaerobic digester allows characterizing, from LCA point of view, the fuel chain of biogas. It is again possible to take advantage of the available inventories from Ecoinvent® database to prepare the model.

As first and most important assumption for this phase, the environmental impacts related to the treatment of wastewater are not included in the model for biogas production via anaerobic digestion. This consideration is related to the fact that biogas at a WWTP is produced anyway as by-product, during water treatments. In the inventories reported any information about wastewater treatment, nor of the sludge used for biogas production is thus included [10].

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Inventories are available at Ecoinvent® both for anaerobic digester infrastructure and operation. These are taken and adapted to match the characteristics of SMAT facility in Collegno.

Anaerobic digester construction inventory

The available data present an inventory for a digester sized for the treatment of 100’000 PCE4 [10]. Using the average wastewater flow treated and the BOD5 content of the water it is possible to calculate the PCE for SMAT WWTP.

Table A7. PCE calculation for SMAT WWTP, Collegno.

Description Value UnitAverage daily wastewater flowa 40’000 m3/dBOD contentb 0.20 g/lDaily BOD load 8’000 kg/d

133’333 PCEa) Data for SMAT Collegno WWTP [11]

b) Data referred to SMAT Castiglione WWTP [11]

Assuming that the type of building is the same, the quantities are then linearly scaled to match the PCE calculated in Table . A time period of 30 years is assumed for the life of the infrastructure. Table resumes the inventory voices for the manufacturing of the anaerobic digester.

Table A8. Inventory for the construction of the anaerobic digester (133’333 PCE).

Materials/energy flows Value UnitConcreteReinforcing steel 64’800 kgChromium steel 18/8 5’253 kgCast iron 1’344 kg

Anaerobic digester operation inventory

The operation of anaerobic digesters is heavily dependent on the temperature, which influences the productivity of biogas and is also a sensible mean of control, since even slight variations could compromise the biological activity, thus slowing the production process. It comes along that these requirements influence the energy demand for the operation of such technology. According to the information available for SMAT facility, the set point temperature for digester operation is taken as 36°C.

During digester operation the sludge must be recycled and heated. The recycling is achieved with a set of electrical pumps, while the heat is provided by boilers.

4 Per Capita Equivalent; equals 60 grams of BOD in raw sludge per day per person.5 Biochemical Oxygen Demand; it is a measure for the content of readily degradable hydrocarbons; it equates to the amount of oxygen taken up in microbial decomposition of hydrocarbons – usually in dark incubation at 20°C and over 5 days.

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Two different operation modes have been considered, the first without thickening of the wastewater sludge (SST = 1,91%) and a second case with thickening of the sludge (SST = 5%). Techno-economic studies and personal communications with SMAT technicians allow the author to compute the total heat and electricity demand for the digester for both operation modes [12].

Table A9. Anaerobic digester energy requirements for operation.

Without thickening (actual situation) Value UnitHeat demand a 3’454 MWhth/yElectricity demand (sludge recycling)a 184 MWhel/yWith thickening Heat demand a 1’642 MWhth/yElectricity demand (sludge recycling) a 184 MWhel/yElectricity demand (thickening) b 130 MWhel/ya) Data from SMAT facility in Collegno.b) Calculation based on tech specs of commercial thickening machine and average sludge flow.

From Table it is evident that the energy requirements for heating could be drastically reduced with the adoption of a simple machine for thickening the sludge. In fact, this option would cut heating demand about 53%; on the other hand electricity consumption increases about 70%.

Another aspect related to the thickening machine is the need of a quantity of polyelectrolyte (special polymers used for concentration of high water content sludge) for the operation. Due to lack of available information about this material, it is not included in the inventory.

Other inventory voices included in the reference model regard lubricating oil consumption and the emissions of CO2 and CH4 due to leaks in the system. For these quantities, the values are scaled based on the productivity of biogas of the plants.

Table and A11 report the inventories for both scenarios of digester operation.

Table A10. Inventory for the operation of the AD W/O sludge thickening (reference: 1 Nm3 of produced biogas).

Materials/energy flows Value Unit

Heat, natural gas, at boiler condensing modulating a

Heat, biogas, at SOFC 250 kW a 5.81 MJElectricity, low voltage, at grid IT 0.26 kWhLubricating oil 2.85E-04 kgAnaerobic digestion plant 4.71E-08 pEmissions to air

Carbon dioxide, biogenic 0,13 kgMethane, biogenic 4.34E-03 Kga) Part of the heating demand is covered by the SOFC, according to its productivity, the rest is provided by the boiler.

Table A11. Inventory for the operation of the AD with sludge thickening (reference: 1 Nm3 of produced biogas).

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Materials/energy flows Value Unit

Heat, natural gas, at boiler condensing modulating a

Heat, biogas, at SOFC 250 kW a 5.81 MJElectricity, low voltage, at grid IT (digester) 0.26 kWhElectricity, low voltage, at grid IT (thickening) 0.18 kWhLubricating oil 2.85E-04 KgAnaerobic digestion plant 4.71E-08 pEmissions to air

Carbon dioxide, biogenic 0.13 kgMethane, biogenic 4.34E-03 Kga) Part of the heating demand is covered by the SOFC, according to its productivity, the rest is provided by the boiler.

CO2 recovery: microalgae production via tubular photobioreactor

The SOFCOM proof-of-concept plant used as base model includes a tubular photobioreactor for carbon capture and algae growth. PBR is a novel technology and it is very difficult to get detailed information, required for a proper inventory for a working plant at industrial scale (experimental installations are more diffused, but they are not suitable for modeling industrial scale plants). The inventory built for this study thus relies on literature data and not every life cycle phase is modeled due to lack of information.

In particular, lack of information about the design and manufacturing of tubular PBRs at industrial scale makes it impossible to model this phase, so it is left out of the study. Instead, the attention is here focused only on the operation of the PBR, for which an inventory is compiled.

The present study does not assess directly the production of fuels from algae, but instead focuses on their ability to fix CO2 into biomass. In addition, the installation of these systems within the context of WWTPs leads to a third positive effect, which is the purification of wastewater from nitrates and phosphates, both being nutrients needed, along with the CO2, for algal growth.

To date, many studies assess the potential of this and similar technologies, for biofuels production and CO2 capture. However, deep understanding of the true potential of these systems is not achieved yet. Even if they seem to provide a win-win situation, both capturing CO2 and producing a useful substrate for biofuels production, technical issues related to the plants for algae growth may result too big, impairing their apparently very positive contribute to sustainability [13].

The design chosen for the demo plant refers to horizontal or inclined tubular photobioreactors. These are composed by a solar field made by arrays of horizontal, parallel tubes, where algae and medium (i.e. wastewater) are re-circulated through a loop. The advantage offered by geometry is the availability of optimizing exposure to light, maximizing biomass production.

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The feed CO2 is injected via dedicated gas exchange system, thus it is possible to control the feeding according to the biogenic activity. The oxygen produced by the photosynthetic activity causes the reduction of biomass production, so it should be controlled as well and, eventually, eliminated. The most relevant drawback, though, is related to the high energy consumption, reported around 2000 W/m3 in respect to the 50 W/m3 achieved by other photobioreactors [14]. This consumption is necessary to keep high linear velocities in the tubes (20-50 m/s), required to maintain turbulent conditions.

Photobioreactor operation inventory

For the PBR no information is available at Ecoinvent® database, hence it is needed to build a new inventory. The first approach was to consider that all the CO2 emitted at the fuel cell had to be captured and sent to the PBR. Immediately arises the technical issue of the storage of the carbon dioxide, since the level of operation of the SOFC is practically constant over the day, while the PBR consumes CO2 only by day, and with a rate related to the radiation arriving on the tubes. Moreover, the plant daily productivity changes seasonally. It is decided to size the storage facility on the daily production of carbon dioxide at the SOFC, making a trade-off between the two extremes, full capture and online-only capture (no storage, the CO2 is fixed only when both PBR and SOFC are functioning).

The next core data regards the fixation of CO2 in form of algae and the productivity of the plant, to become able to compute a yearly biomass production, collected from reports related to Bioalma (www.bioalma.polito.it) and personal communications from MATGAS (www.matgas.org) experts. Firstly, the amount of carbon dioxide which is possible to fix in biomass is strictly related to the carbon content of the algae. It is said that algae carbon content is around 50% of dry mass (from SOFCOM tests). This said it is possible to calculate a conversion factor from CO2 to biomass of 1.83 kg of CO2 per kg of algae.

The next value to be set is the productivity of the PBR: for the present work it is assumed as 30 g/m2/day. This value is aligned with literature productivity for the same technology, reported as 30 g/m2/day both from Bioalma (www.bioalma.polito.it) and in [15].

Table A12. Preliminary sizing of PBR and CO2 storage facility.

Description Quantity UnitDaily CO2 emissions 3.44 ton CO2/dMax daily biomass production 1.88 ton/dTotal surface needed 6 haEquivalent soccer groundsa 8 CO2 storage (for 1 day SOFC operation)

1’737 m3 (ambient pressure)174 m3 (10 bars)

a) 1 soccer ground = 110*75 m2.

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Starting from the mean yearly value for productivity of 30 g/m2/day, it is possible to obtain monthly values of productivity scaling this mean value using the average monthly irradiation available at the site. The meteorological information are retrieved at ENEA’s Atlante Italiano della radiazione solare (www.solaritaly.enea.it).

CO2 storage has been sized on the daily production from the SOFC system. Because of the limited CO2 captured, compared to the potential consumption to maximize the PBR productivity (according to the availability sunlight in Turin), the actual algae production is lower than the potential one for the chosen PBR and sunlight data.

Moreover, not all the CO2 injected in the PBR is fixed as biomass; a part of it passes through the system and is then emitted in the atmosphere. According to the information available in the project Bioalma (www.bioalma.polito.it), the molar fraction of CO2 in the injected gas to the PBR plays an important role in determining CO2 losses. Due to lack of more detailed information about this aspect, a constant value of CO2 fixation is taken as 75%, as reported in [15].

The average daily values of potential (CO2 always available, no losses) and computed biomass productivity for every month are shown in Figure A2.

The blue curve represents the potential of PBR productivity, namely the productivity in the case of total fixation of the CO2 introduced in the system (fixation yield 100%) and with no restriction of nutrients. The red curve, instead, represent the values of daily productivity computed taking into account the losses of CO2 during operation (fixation yield 75%) and the limitation of productivity related to the availability of CO2 during the year. During winter, the production is limited by the low available radiation, while during the rest of the year the production is restricted by the available CO2 from the SOFC.

The total production of algae is calculated as 410 tons per year. Using this result, it is evaluated the level of CO2 sequestration, approximately 60% of the total emissions from SOFC.

Having all the nutrients already available makes WWTP an ideal site for the installation of such technologies.

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Figure A2. PBR potential and computed productivity. Computed productivity is limited by the availability of CO2

and by losses (fixation yield 75%).

As for the energy requirements, energy consumption in PBRs is mostly related to electricity consumption for the hydraulic pumps employed for feeding and recirculating water and algae. This electricity consumption strictly depends on plant configuration, but it is possible to gather representative values from literature studies on the same kind of plants. A value of 1 kWh/m2/day is chosen, following [15] and [16].

Other energy consumption would occur for successive processing of algae, such as harvesting, but since the use of biomass is not included in the study, it is not considered.

CO2 emissions are calculates as the difference between the total yearly emissions from SOFC and the amount of CO2 fixed in biomass.

Table A13. Inventory for PBR operation (reference: 1 kg of algae).

Materials/energy flows Value unit

Electricity, medium voltage, at grid IT 56 kWh

Emissions to air

Carbon dioxide, biogenic 1.23 Kg

From Table emerges immediately that growing algae using this configuration is very energy intensive. To point out this fact, a simple calculation of the energy efficiency of the process is performed. This efficiency can be defined as the ratio between the energy contained in the algae (LHV) over the energy consumption of the PBR. Algae LHV is taken as 21.1 MJ/kg, following the value available at Bioalma (www.bioalma.polito.it).

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ηPBR=algae productivity∗LHVelectricity consumption

=10.5 %

The implications of such low energy efficiency are thoroughly studied in LCIA phase.

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