Fuel cells (FCs) are expected to play an important role in reducing environmental burdens associated with energy conversion technologies to achieve the current EU objectives [1
]. Fuel cells are particularly interesting due to their high efficiency, modularity, excellent partial load performance, low pollution emissions and possible integration with other systems (e.g., steam or gas turbines) [2
]. Solid oxide fuel cells (SOFCs) are suitable for distributed stationary power generation because of their fuel adaptability (they can employ a large variety of hydrocarbon fuels), the possibility of partial load operation and the possibility of cogeneration (heat recovery).
For sustainability evaluations, various policy documents underline the need of accurate information related to the environmental performances of products and service, especially in case of the introduction of innovative technologies on the market [6
]. To assess the environmental sustainability of a product/service/new technology, a life cycle approach should be adopted to guide policymakers and consumer decisions and to introduce innovative sustainable technologies on the market [6
]. Among the tools available to assess the environmental impacts of new technologies, Life Cycle Analysis (LCA) is a standardized methodology [9
] widely used by the scientific community.
Large scale fuel cell systems have received growing interest in the scientific world and the market. Nonetheless, LCA of such systems is not straightforward and rarely available. Only a few studies deal with the LCA of real operating fuel cell plants.
Jing et al. [12
] have developed a multi-optimized SOFC model evaluating, for a specific case study, environmental and economic benefits. When authors are talking about environmental analysis, they are mostly referring to emissions analysis. Life cycle analysis is indeed a comprehensive study able to evaluate the impact of a specified system over its entire lifetime. A recent study from Benveniste et al. [13
] deals with the LCA of micro-tubular SOFC for auxiliary power units (APUs) fed by liquefied propane gas (converted into hydrogen in a dedicated catalytic reformer before being sent to the fuel cell): results show a reduction of 45% in terms of CO2
equivalent emissions and 88% in terms of Primary Energy consumption compared to conventional Diesel APU systems. Furthermore, the work points out that Global Warming Potential (GWP) and primary energy impacts could be reduced by reducing the energy consumed during the manufacturing phase and improving the system efficiency (operative phase).
The European Project FC-Hy Guide [14
] has extensively used life cycle assessments to better understand engineered solutions towards more environmentally sound fuel cell production and use. A guidance manual for LCA application to FC technologies and systems has been developed and contains essential information on how to build LCA of hydrogen-based and fuel cell technology, with details on the processes to be included, the approach, the steps and inputs/ outputs of the system [15
]. FC-Hy Guide does not include a real case study application of the proposed method with SOFC, which is indeed developed in the presented work. The project has analyzed, in a published work [14
], the LCA of a Molten Carbonate Fuel Cell (MCFC). The analysis shows a non-negligible impact, especially in GWP and abiotic depletion categories, of the fuel feeding the system (NG in this case) [14
]. As far as the FC module manufacturing and operation is concerned, it instead affects acidification, eutrophication, photochemical oxidation, ozone layer depletion and human toxicity categories. Among the different components included in the MCFC system, the reformer is the most impacting in almost all categories, because it requires palladium and platinum catalyst, followed in impact by the power conditioning system. The use of a renewable gas feed (such as biogas) would help in reducing the fuel impact; furthermore, the reformer could also be avoided if green hydrogen from renewable sources would be chosen as fuel.
Despite the critical aspects shown by the previous work on MCFC, other studies on the LCA analysis of such systems show benefits compared to traditional technologies like microturbines [16
]. Staffell et al. analyzed energy consumption, process-related emissions and carbon payback time of Combined Heat and Power (CHP) systems based on alkaline fuel cells or solid oxide fuel cells [20
Other work available in the literature is related to polymer electrolyte fuel cells (PEMFCs) because of their interest for the automotive sector. Evangelisti et al. [21
] compare an FC vehicle with an ICE-based vehicle and a battery electric vehicle. The production process showed a higher environmental impact for the FC vehicle compared to the production of the other two vehicle’s power sources (and due to the hydrogen tank and the fuel cell stack). A potential reduction of 25% in the climate change impact category for the FCEV has also been detected when moving from the current scenario to an optimized one, with more enviromentaly friendly components (especially the hydrogen tank and the PEMFC stack). Over the entire life, ICE-based electric vehicles show the worst performance indeed because of fossil fuel use during use phase. One option to reduce environmental impact in terms of, for example, ADP of FC-based cars is the option of platinum recycling at the end of life, as analyzed by Duclos et al. [23
]. Their work shows that more than half of the main impacts of the membrane-electrode-assembly can be avoided for four relevant impact categories if platinum is recovered at the end-of-life of the product.
A similar state-of-the-art knowledge on LCA is also available—even if with a smaller number of contributions—for SOFCs: different works are available and deal with the various fields of applications of SOFC technology: APU [13
], micro-CHP, large-size CHP, building sector [24
]. Longo et al. [25
] have analyzed LCA of PEMFC and SOFC in the book Hydrogen Economy
, edited by Academic Press; here the authors provide a literature review of available LCA researches to point out the environmental impacts of the FCs. Mehmeti et al. [26
] published a recent (2016) work reviewing the state of the art of LCA in SOFC systems. This is one of the most comprehensive works on the state of the art of SOFC systems.
Few works are available in the literature focused on the SOFC application in cogeneration mode in industrial plants. Tonini et al. [27
] analyzed the biomass-based energy system in Denmark using LCA tool. The authors analyzed future scenarios (2030 and 2050) by introducing innovative energy system for transport fuels supply. SOFC, fed by biogas and syngas was used for electricity production in future scenarios. Thanks to the combination of the different technologies involved, the authors found a reduction ranging from 66 to 80% in GHG emissions.
Sadhukhan at al. [28
] performed a comparison between biogas-fed SOFC, PEMFC, micro-GT and ICE in terms of environmental performance: in terms of avoided GWP, Acidification Potential (AP) and Photochemical Ozone Creation Potential (POCP), biogas based PEMFC microsystem is depicted as the most beneficial compared to the equivalent natural gas based systems. End-of-life management of SOFC materials is also another un-explored area, which could lead to interesting scenarios.
Life cycle assessment of biogas plants, without the use of innovative fuel cell systems, has been deeply studied in the literature. Recent studies focus on the comparison of different biogas exploitation paths in specific countries, like Malaysia—where a huge potential for biogas from palm oil biomass was found [29
]—and Nigeria, where biogas from organic fraction of municipal solid waste was found [30
]. Garfí et al. [31
] evaluated the installation of small-size digesters for biogas production in Colombian farms: a potential environmental impact reduction up to 80% is associated with manure handling, fuel and fertilizer because of the biogas production. The same concept was demonstrated—through an environmental analysis—for Ethiopia by Gabisa et al. [32
] and for Bangladesh by Ali et al. [33
]. More recent and general reviews on LCA of agro-biogas are also available in literature [34
]. Dedicated energy crops cultivation for biogas production has been evaluated by Torquati et al. [36
]: crops production indeed plays a crucial role in the whole process LCA.
Most of the works related to the LCA of SOFC systems [37
] are referring to the same databases when dealing with the SOFC manufacturing inventory. One of the central criticality of data collection on SOFC production is that there are not many companies worldwide, which are manufacturing SOFC systems at industrial scale. The novel aspects of the present work is the choice of recent and updated sources for data collection, both in terms of SOFC production and operation; in particular:
For what concerns the SOFC manufacturing phase, a 2015 report from Ernest Orlando Lawrence Berkeley National Laboratory is used [40
]. Thanks to the cooperation with the worldwide largest SOFC manufacturers, the report analyzed SOFC applications for use in CHP and power-sector only from 1 to 250 kW-electric. The resulting total cost of ownership includes the direct manufacturing cost, operational costs, and life-cycle impact assessment of possible ancillary financial benefits during operation and at end-of-life. The report provides data on an industrial production of SOFC systems, which is difficult to find in literature and is available thanks to the laboratory cooperation with FC producers.
For what concerns the operation phase and the SOFC management in a real industrial environmental, data have been retrieved from the DEMOSOFC (Demonstration of large SOFC system fed with biogas from WWTP) plant the first industrial-scale installation of a biogas-fed SOFC plant in Europe. The three SOFC modules, supplied by Convion [41
], produce about 174 kWel
and around 90 kW-thermal. All the generated energy is self-consumed within the Waste Water Treatment Plant (WWTP) of Collegno (Torino, IT), where biogas is produced from sewage sludge. Two SOFC modules are currently running since October 2017. The use of real data represents a unique and significant added value for the LCA study.
This work thus assesses the potential environmental impacts of a CHP plant that employs medium size SOFCs, fed by biogas produced by a WWTP facility, with a life cycle (cradle to gate) approach. The first section is related to the methodology presentation, the scenarios definition and the Life Cycle Inventory (LCI) (Section 3
, Section 4
and Section 5
), which discuss all the input data. Then, Section 6
shows and discuss the results. The primary goal of this study is the characterization of the energetic and environmental burdens of the three WWTP case studies through sustainability and life cycle impact indicators. The LCA developed in this work is comparative, so benefits or disadvantages are relative to the reference scenario (Scenario 1).
2. Plant Layout and Scenarios Definition
A WWTP is mainly divided into two sections (Figure 1
): (1) a water line, in which wastewater undergoes to physical, biological and chemical treatments in order to meet the thresholds imposed by the existing standards; (2) a sludge line, where the organic matter separated during water purification is pumped towards the anaerobic digester. During the anaerobic digestion, microorganisms break down the organic substance contained in the sewage sludge and partially convert it into biogas. A WWTP needs electrical and thermal energy to sustain all these processes [42
Three different scenarios for the WWTP are presented:
Scenario 1: the reference scenario in which all the electricity needed for operations is purchased from the grid and biogas is exploited in a boiler for thermal recovery or flared. No CHP system installed, and this represents the ante-DEMOSOFC scenario.
Scenario 2: it foresees the installation of the SOFCs CHP system and biogas management improvements (since biogas is primarily sent to the CHP system and surplus gas, when available, is still used for thermal production in the existing boilers).
Scenario 3: is similar to the second one but with an improvement in the anaerobic digestion line. A dynamic sludge pre-thickening machine is indeed employed to reduce the thermal demand of the anaerobic digester [44
The WWTP analyzed in this work is sited in Collegno, a municipality within the metropolitan area of Turin, Italy [49
]. A brief description of the integrated plant layout is useful to understand the primary energy and mass inputs/outputs of the system. The focus is on sludge and biogas lines since they are affected by the installation of the SOFC-CHP system within the wastewater treatment plant.
In Scenario 1 (Reference) (Figure 2
), raw and activated sludge produced during wastewater treatment are pre-thickened in separated tanks exploiting gravitational forces. Secondary sludge is treated with ozone to reduce the total amount of sludge volume to be processed. Although ozonization is not the best option for what concerns anaerobic digestion yield—biogas produced per capita is lower respect to other plants—it is an optimal process from the overall plant since it reduces the total amount of sub-products. Raw and activated sludge are both heated before entering the digester, which works in a mesophilic range of temperatures (35–45 °C). Part of the sludge and the produced biogas is continuously re-circulated in the tank to maintain high renewable-gas yield. The digested sludge is sent to a post-thickener, a press filter, to reduce the water content and make it available as fertilizer. The presence of a gas holder is fundamental to manage sludge and biogas production fluctuations, due to variable wastewater intake. The only use of biogas in this research is in boilers for producing the thermal energy needed for self-sustaining the anaerobic digestion process. Thermal demand of the anaerobic digester is equal to the sum of the energy required for sludge heating (up to set point temperature, ~42 °C) and that required to compensate losses through walls and pipes. Biogas in excess is flared. When biogas flow is not sufficient, the thermal demand is satisfied by natural gas taken from the network and feeding the boilers. The whole amount of electricity is purchased from the grid. Annual electrical and natural gas consumptions and average biogas yield and production rate are provided by the owners of the plant (SMAT, Società Metropolitana Acque Torino, [49
In Scenario 2, the installation of a not-conventional CHP unit improves the WWTP energetic self-sufficiency. Its very high electrical efficiency, and the operation in CHP mode are the motivation for the choice of the SOFC technology. Its adoption in the project is oriented towards its market introduction on an industrial scale using a demonstration of its energetic and environmental performance [50
]. SOFCs generate electricity directly from the chemical energy contained in the biogas, with high efficiency and near-zero emissions of pollutants (e.g., CO, NOx,
and hydrocarbons). The disadvantages are fuel cell sensitivity to biogas contaminants (in sewage biogas mainly sulfur and silicon compounds) and to thermal cycles (shutdown should be avoided). As shown in Figure 3
, three main sections represent the change in infrastructure in the WWTP:
The biogas processing unit, where biogas is dehumidified, cleaned from harmful contaminants and compressed;
SOFCs cogeneration modules (total power 174 kWel), where electrical energy is produced and used for internal plant needs;
Heat recovery section, where thermal power contained in exhaust gas exiting from SOFCs is recovered and transferred to the sludge entering the digester;
Biogas handling is changed since now its primary goal is feeding the CHP modules while the surplus is sent to boilers to satisfy digester thermal demand.
Moreover, as in the reference case, biogas in excess in the gas holder is burned by the flare system. When the amount of biogas in the gas holder is not sufficient to cover digester thermal demand, natural gas is withdrawn from the grid. In this second scenario, the electrical consumption of the WWTP is higher, owing to absorption of the power of some components in the balance of plant (e.g., biogas compressor, chillers, and control system).
Scenario 3, in which the SOFC CHP unit is still present, foresees a reduction of the thermal demand for the anaerobic digestion process through an increase of the level of thickening of sludge (dry matter from 2.7% to 6.4% in weight) [51
The use of a pre-thickening system for the inlet biomass to the digester is a strong WWTP optimization because it enables the plant to install high efficiency CHP systems while keeping self-sufficiency on the thermal power side. The sludge stream entering the diester has a very low solid content (usually around 2%), and this generated a huge request of thermal power for pre-heating the flow from ambient to digester temperature. In case of an SOFC installation, thermal power production is reduced compared to the baseline (because of the electrical production) and is not anymore enough to cover the thermal load (and extra NG from the grid is required, thus increasing the fossil fuel consumption). When a pre-thickening system is installed, solid content is increased up to 5–8%, and thermal power request is reduced. In this optimized scenario, the SOFC thermal production is able to almost fully cover the thermal demand of the digester, thus reducing/deleting the consumption of NG from the grid.
At the same time, the installation of a dynamic thickening machine is responsible for a slight increase in electrical consumptions of the WWTP.
summarizes the resulting shares of electrical and thermal energy coverage and the biogas handling with the plant. Input data for the development of the energy balance are:
SOFC electrical efficiency: 53.1% [41
SOFC thermal efficiency: 25.8% [41
Yearly equivalent capacity factor: 95% (assumption)
Ordinary maintenance per year: 7.5 days (assumption)
Digester thermal load (daily-based) definition as described in [52
Electrical load (monthly-based) from SMAT data. Average yearly consumption equal to 20.88 kWh/PE/y, in line with the work developed by Panepinto et al. on a similar SMAT-owned WWTP [45
Boiler efficiency: 90%
Biogas average macro-composition: 60% CH4–40% CO2
The onsite experience within the DEMOSOFC project is the source for the assumptions on the number of days for the ordinary maintenance and the yearly equivalent capacity factor. The only required yearly ordinary maintenance on the SOFC modules is the replacement of the air inlet filters and—on 1–2 years basis—the reformer catalyst replacement.
As can be seen from Table 1
, in scenario one all electricity is purchased from the grid and heat is supplied mainly by biogas (with an NG contribution only in winter season). In Scenario 2, around 25% of the electrical energy is self-produced thanks to the installation of the SOFC system. Thermal energy provided by NG is increased (from 7 to 45%), because of the use of biogas in the CHP unit. This criticality is solved in the third scenario where electricity share is equal to the second one, but the thermal load is reduced (thanks to the installation of a sludge pre-thickening system) and consequently NG consumption is zero.
For each scenario previously introduced, the unit processes included in the boundaries are analyzed, and the compilation of all the relevant input/output flows (concerning the functional unit) is performed. Figure 4
and Figure 5
show that Scenario 1 (reference), in which biogas is exploited only in boilers for thermal power production, operational phases associated with the WWTP itself are part of the inventory. For Scenarios 2 and 3, in which a cogeneration system is installed in addition to the existing boilers, the analysis also includes the manufacturing and the operation and maintenance of the SOFC-based CHP system.
4.1. SOFC Stack Manufacturing
A solid oxide fuel cell is a device allowing the direct conversion of chemical into electrical energy, at high temperature. A single cell consists of three layers, a dense electrolyte between two porous electrodes (anode and cathode). Because of limitations in single cell voltage, the cells are connected in series to form a stack using interconnector plates, manifolds, flow fields, and sealant. This unit process is analyzed in detail since it is the core of the CHP system and innovative materials are continuously tested and employed to improve the overall efficiency.
A detailed work developed at the Lawrence Berkeley National Laboratory has been the source of information on fuel cells manufacture [40
]. The design and manufacturing steps of the SOFCs closely follow those of Fuel Cell Energy Inc., which has acquired Versa Power System. Table 2
shows the geometrical and functional characteristics of the selected SOFC stack.
It is essential, whenever a manufacturing process is analyzed, to fix the production volume to normalize material and energy flow respect to a reference unit, in this case, a single stack. From [40
] it has been chosen a production volume of 50,000 stacks per year equal to 32,500,000 electrode-electrolyte assembly (EEA) cells per year. Another important aspect associated with a manufacturing analysis is the determination of line process parameters (e.g., line availability, performance, and yield), which are linked to the level of automation and the annual production volume of the site.
The part of the cells in which electrochemical reactions occur is the electrode-electrolyte assembly (EEA) which is planar, and anode supported. The anode is tape casted while the other layers are deposited on the support by screen printing machines (see Table 3
With a single step co-firing all layers are sintered together in a kiln. The set of processes included in the EEA manufacturing analysis are slurry preparation, ball milling, de-airing and pumping, tape casting, screen printing, first quality control, co-firing, laser cutting and final quality control.
SOFC interconnectors are made of a stainless steel alloy (stainless steel 441, composed of 17–24% of chromium) to maintain the right physical property at elevated operating temperatures. A manganese cobalt spinel oxide is physically vapor deposited and used as a protective layer to avoid chromium poisoning of the cathode. The processes involved in the interconnector manufacturing are stamping, cleaning and drying, PVD (Physical vapor deposition) of the coating and final inspection. SOFC frames are made of the same materials of interconnectors, and their manufacture foresees the use of analogous machines.
The seal is needed to prevent mixing and leaking of fuel and oxidant within/from the stack and to provide electrical isolation of cells and mechanical bonding of components. Planar SOFCs are usually jointed by means of glass seals. Cell to frame seal is applied for the cell to frame joining. Steps involved in the sealing process are ball milling of the glass paste and heating under a static load in a furnace. A semi-automatic stack assembly line is stacking up repeat units, and attaching current collectors or end plates to both ends of each stack. A final fully automated conditioning and the testing station is monitoring physical, chemical and electrochemical properties and performance. Table 4
shows the input data, where the reference unit is the manufacture of one stack of 10 kW nominal net power.
Among the EEA manufacturing processes, the most energy intensive is co-firing which is responsible of around 73% of electrical demand. The total electrical consumption is 1083 kWh per stack manufactured (so around 108 kWh/kW), and a graph of contributions of processes is shown in Figure 6
. Air emissions are related to the preparation of the slurry and the complete evaporation of solvents in the drying step. Carbon dioxide emissions are taken and scaled from [56
A comparison with a merged inventory taken from literature [20
] is performed to check the reliability of acquired data. This study is quite old and analyses a different type of fuel cells (electrolyte-supported EEA). Nevertheless, there is a reasonable agreement between Versa Power and literature data.
4.2. CHP System Manufacturing
The DEMOSOFC plant comprises of three C50 modules. The C50 is an SOFC power generator with a nominal power output of 58 kW (AC net) (Convion [41
]). Thanks to its modular architecture, multiple units can be installed to achieve higher power outputs. Each module includes several SOFC stacks, a biogas pre-reformer, an afterburner, fuel, and air heat exchangers, blowers, air filters, start-up components (e.g., electrical resistance), control system, piping and valves, and casing. Since no specific information on materials and energy needed for manufacturing a C50 module are available from Convion, literature has been revised to find data on some of these components [40
]. A general description of the balance of plant is useful to understand the compilation of inventory provided in Table 5
Biogas exiting the gas holder to feed the CHP units flows firstly through a recovery station, which comprises of a blower and a chiller, to have enough pressure to reach the treatment zone (positioned in another part of the WWTP) and avoid water condensation. In the biogas treatment section, filtration, compression, dehumidification, and post-filtration are performed to satisfy the strict purity requirements imposed by SOFCs (S level below 30 ppb, and total Si below 10 ppb). With the aim of improving the reliability and continuity of operation of the cleaning system, a lead and lag configuration is employed [50
]. The clean-up reactors are adsorption vessels containing types of activated carbons specific for siloxanes and sulfur removal. Separated and dedicated feeding lines transport the purified biogas to the three SOFC modules.
Thermal recovery from C50 modules is performed using two interconnected loops. The use of a secondary water-glycol circuit is essential to avoid fouling of heat exchangers inside the CHP units due to the dirty stream of sludge involved. Therefore, heat released by hot exhaust is transferred to the water-glycol mixture and then to the sludge directed towards the anaerobic digester. As previously said, based on the amount of thermal energy available from CHP units, a certain amount of sludge can be pre-heated by the SOFC, while the remaining part is heated up through the conventional hot water loops of boilers, which are fed by extra-biogas available in the gas holder or by natural gas from the network.
The three C50 modules are connected to the grid. During start-up, the fuel cells absorb power from the grid, while during nominal operation power is exported. The connection of the SOFC modules with the external grid foresees medium voltage switchgear that is connected using transformers to the low voltage one. DC produced by SOFC must be converted through inverters in AC.
As it is easily understood, the analyzed balance of plant includes many components, and it is not possible to perform a detailed data collection for each of them. Rough but at the same time necessary approximations are performed when compiling the inventory. The path chosen is to scale, update and modify datasets of similar systems available in other studies [56
] according to the size of the analyzed plant.
Since C50 unit has a rated electrical power of 174 kW and in the WWTP three modules are installed, a total amount of 18 stacks (10 kW each, according to the initial assumptions) is considered when compiling the inventory. Inside the modules, a material flow that cannot be neglected during data collection is the catalysts present in steam reforming (SR) and water gas shift (WGS) reactors. These components convert the methane contained in biogas to syngas before feeding the anode of SOFCs. The SR reaction is strongly endothermic and creates more gas volume as the hydrocarbon is converted. This means that high temperatures and low pressures favor it. Instead, WGS reaction is slightly exothermic, so it is supported by low temperatures. Both reactions are catalyzed to improve methane conversion and decrease the risk of carbon formation. Several parameters influence the choice of the catalyst: primarily activity and cost but also the potential for carbon formation, heat transfer, strength and packing properties, pressure drop during operation [59
]. Modern catalysts are for the most part made of supports onto which the active metal is impregnated. In this study, it has been supposed that the reactors use catalysts composed of 63% of alumina, 20% of nickel and the rest of silicon for steam reforming and iron for water gas shift. Information about the amount of catalysts employed is taken from [60
], by scaling available literature data based on biogas flow to CHP modules. The same amount of catalyst in SR and WGS reactors has been assumed.
All the other components of a C50 module are assumed made of stainless steel since they operate at high temperatures. A single module weighs six tons, and the amount of stainless steel has been determined by subtracting the mass of stacks and catalysts.
Concerning the fuel processing unit, the clean-up filtering media have been modeled. Activated carbons are employed as adsorbent materials for sulfur, siloxanes and VOC (Volatile organic compounds) removal. Activated carbons (AC) can be manufactured from a variety of raw materials that have a high percentage of carbon content and low impurities. Activated carbons are characterized by a very high internal surface area. In the four tanks dedicated to siloxanes and VOCs removal, non-impregnated steam activated carbons produced from coal are used. The amount of filtering media needed per bed has been calculated scaling data from [62
] as a function of biogas flow rate. Some parameters affect the quantity of filtering media used, such as operating temperature and pressure and level of purification pursued.
The other mechanical components of the biogas processing system and the heat recovery section are considered in terms of the equivalent amount of reinforced steel. For the SOFCs CHP system, a specific weight of 200 kg of steel per installed electric kW is taken from [58
]. Making a difference with the weight of C50 modules, the BoP (Balance of plant) result composed of around 16.8 tons of reinforced steel. The electric system is modeled with the number of inverters of 2.5 kW needed to reach total power (174 kW). The electrical and thermal energy required for CHP system production and assembly is taken from [58
] and scaled based on the power plant size. As said, these rough simplifications are necessaries since specific data from manufacturers, or suitable datasets in databases for some components of the BoP, are not available.
4.3. CHP System Maintenance
In this life cycle phase, all the necessary replacements of parts and consumables are considered. It is assumed a six years lifetime for the SOFC. Concerning the activated carbons, each adsorption vessel in lead position will reach saturation after six months of continuous operation, so that two replacements per year are required. The catalysts of SR and WGS reactors are entirely replaced every four years. Other maintenance requirements (e.g., malfunctioning parts, occasional damages) are modeled as substitution of steel corresponding to 1% of the total mass in the system. Primary data are reported in Table 6
4.4. CHP System Operation
Reference flows are thermal and electrical energy produced by SOFC modules in one year. Since the CHP system was not operational when the analysis was performed, the simulation of plant performance is achieved through a tailored energy planner tool [63
]. The installation in the WWTP of an SOFC CHP system implies the determination of smart and efficient management of biogas stored in the gas holder. For the scope of this work, it is enough to say that the primary aim is to avoid fuel shortages and to minimize SOFC shutdowns during the year. This goal is reached using a regulation of the SOFC power output according to the monitoring of the gas holder level. In Table 7
the most important operational parameters, obtained from the simulation, associated with the three SOFC modules, are reported. In the calculations, a constant percentage of methane of 60% is considered in the biogas and a corresponding lower heating value of 21.5 MJ/Nm3
The multi-functionality issue associated with the production of heat and electricity by the CHP units is solved through the allocation based on exergetic contents of these streams. In Table 8
the inventory associated with CHP system operations is shown. The amount of system necessary for one year of operation is calculated as the inverse of plant lifetime, assumed of 20 years.
4.5. Boilers Operation
As already said, thermal energy is requested to maintain the anaerobic digester in an optimal range of temperatures, to maximize biogas yield of the process. The exhaust gas analysis, and so the emissions associated with combustion, have been provided directly from maintainers of the plant. The amount of biogas and natural gas (NG) burned in boilers changes among different scenarios, so separated inventories have been produced in Table 9
. The common reference flow is the amount of heat delivered in one year of operation.
4.6. Anaerobic Digester Operation
The digestion process requires thermal energy, but also electricity for sludge mixing and recirculation. The processes to which wastewater is subjected to obtain raw sludge, as well as the subsequent treatment of the digested matter, are outside of the boundaries of the study since they are common phases of different scenarios. Carbon dioxide and methane emissions are due to leakage of pipes during the process and are assumed to be 0.75% of produced biogas according to [66
]. The reference flow is the annually produced biogas; collected data are reported in Table 10
4.7. WWTP Operation
This unit process includes electrical consumptions associated to plant operations, and emissions associated with biogas in excess, which is flared. It is assumed that the whole amount of methane burned is oxidized and converted in carbon dioxide (and water) since no specific information on emissions is available. The functional unit is the amount of wastewater treated by the WWTP in one year; collected data are reported in Table 11
Three alternative scenarios for biogas exploitation in a medium-sized wastewater treatment plant have investigated in this work about their environmental performances. Real data from an integrated SOFC-WWTP have been retrieved from the DEMOSOFC project for what concerns the operation of the SOFC.
A large amount of electricity required for WWTP operations urges for a recovery of the produced biogas, which is available on-site and could cover much of such demand. By the life cycle assessment methodology, the potential reduction of the environmental burdens of a WWTP, in which efficient SOFC-based CHP modules are installed, is assessed. A thermal energy conservation opportunity that foresees the use of a dynamic machine for sludge pre-thickening enhancement is also investigated.
The operational phase of the analyzed components inside the WWTP has proven to be determinant in all the impact category analyzed. The depletion of non-renewable resources (ADP) is primarily linked to the manufacture and maintenance of the cogeneration units and the tailored balance of plant. In the first scenario, a predominant part of the impact in all the categories is associated with the electricity withdrawn from the grid. The LCIA has shown that producing a substantial share of electrical energy (around 25%) via biogas-fed SOFC cogeneration modules can reduce the environmental burdens associated to WWTP operations in five out of the seven impact categories that have been analyzed in this work: AP, EP, GWP, POCP, and PED. A further reduction of impacts, particularly concerning GWP and PED, is possible by the decrease of the thermal demand of the digester, thus making the system independent from natural gas. In both Scenarios 2 and 3, primary energy and CO2 emissions embodied in the manufacture and maintenance of the CHP system are neutralized by operational savings in less than one year.
The sensitivity of LCIA outputs to a variation of electricity consumption and natural gas supply mixes is relevant mainly in the regional impact categories AP, EP and POCP, but also global ODP. The EU-27 mix has a higher impact than the Italian one because a larger dependence on more polluting fossil sources (coal is still employed in large quantities) and nuclear has been highlighted. It is worth to remember that data of energetic mixes available in the software are of 2009 and in the meanwhile significant changes occurred. Nevertheless, it can be said that the quality of produced electricity, measured in terms of its renewable origins, plays a decisive role in the life cycle assessment of energy-intensive systems. Positive effect on environmental loads of second and third scenarios are confirmed when the EU-27 mixes are used; furthermore, a slight reduction of ODP, compared to the first scenario, is obtained.
Main limits associated to this study are low availability of specific data concerning manufacturing and maintenance phases of the balance of plant that makes necessary the use of some rough assumptions, and the exclusion from the boundaries of the work of end of life scenarios (e.g., recycle or disposal of materials) due to lack of usable information. Anyway, the model could be further refined and improved for future studies.
Pursue of electrical and thermal self-sufficiency of WWTPs through the installation of efficient cogeneration systems, and the careful evaluation of energy conservation opportunities both in sludge and water lines seem to go in the right direction towards better environmental sustainability.