Molten Carbonate Fuel Cell - Hikari

Applied Mathematical Sciences, Vol. 8, 2014, no. 33, 1621 - 1633 HIKARI Ltd, www.m-hikari.com http://dx.doi.org/10.12988/ams.2014.4123

Feasibility Analysis of MCFC (Molten Carbonate Fuel Cell) – Anaerobic Digester Power Plant Marco Buccarella Department of Industrial Engineering - University of Perugia Via Duranti 67 06125 Perugia, Italy Andrea Colantoni, Benedetto Baciotti, Biondi Paolo and Francesca Santoni Department of Agriculture, Forest, Nature and Energy (DAFNE) University of Tuscia, Via S. Camillo de Lellis snc, 01100 Viterbo, Italy Luca Salvati Consiglio per la Ricerca e la sperimentazione in Agricoltura, Centre for the Study of Plant-Soil Interactions (CRA-RPS), Via della Navicella 2/4, 00184 Rome, Italy Copyright © 2014 Marco Buccarella et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract Besides traditional approaches for the valorization of biogas, the possibility of using biogas to run Molten Carbonate Fuel Cells (MCFC) for the production of combined heat and power (co-generation) is showing interesting results. The aim of this study is to assess the benefits and draw-backs of this energy system and to compare it with traditional biogas applications. Both technical and economical analysis of a real case-study are carried out. The selected case-study is a pig manure treatment plant located in Marsciano, near Perugia, Italy. The performance of MCFC operating on biogas is assessed through experimental activities carried out at the Fuel Cell Group laboratory of the University of Perugia (Italy). Every test in which the MCFC has been run with biogas from an anaerobic digester, showed a highly satisfactory operation of the fuel cell. The economical analysis is based on the net present value (NPV) of an MCFC system

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and compared to an internal combustion engine system. Costs for the MCFC system are assumed to be equal to 1500 €/kW, 2000€/kW and 3000€/kW and it has been analyzed the difference between them. Different cases with or without economic incentives and with or without co-generation have been studied. Green certificates (i.e. governmental incentives due to the low emissions) are also taken into account for NPV calculation. Keywords: MCFC, anaerobic digestion, Methane, Hydrogen

1 Introduction The problem of electrical energy generation is a controversial topic which from a long time has attracted and still attracts the interest of many different research sectors. Problematic topics such as scarcity of fossil fuels or high CO2 emissions directly related to worldwide energy consumption (estimated around 25000 million metric tons) lead to investigate new energy sources. In 2004, worldwide carbon dioxide emissions from energy consumption have been estimated around 25000 million metric tons [Lens et al. 2005, Friedrich el al. 1999, Colantoni et al. 2013]. This huge amount of emitted carbon dioxide represents a strong threat for the environment and has pushed research efforts to the application of renewable energy sources [Kivisaari et al 2002, Hamdi et al 1996]. One of the possible renewable energy can use for the electric energy production is biogas. The biogas composition present a remarkable amount of methane, thus the energy content of these gases is considerably interesting for energy recovery. Due to the large amount of fossil fuels employed for electricity and heat production, the use of biofuels is especially interesting for these applications. Different technologies are currently available for combined production of electricity and heat. Even if internal combustion engines (ICE) represent the most employed technology due to its high reliability widely demonstrated for decades, fuel cells is a very promising one. Several system solutions have been proposed for integrating fuel-processing systems and MCFC [Lunghi et al 2001, Lunghi et al 2002, Monarca et al 2011, Colantoni et al 2013, Colantoni et al 2011] and the resulting performances are very high compared to traditional systems for energy conversion of biogas. The main goal of this work is focused on the production of electric power and heat through a MCFC system fed by biogas from anaerobic digestion. The main advantages of this system are: • Low environmental impact; • High efficiency; • Possibility of co-generation; • Size flexibility; • Replacement fossil fuels.

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2 Materials and methods 2.1 Overview of anaerobic digestion Among the different existing methods to produce biogas, the most spread one is the Anaerobic Digestion. Anaerobic Digestion (AD) [Dennis et al 2001, Salminen 2002, Lens et al 2005] is the conversion of organic material by a microbial population that lives in an oxygen free environment. When organic matter is decomposed in an anaerobic environment the bacteria produce a mixture of gases composed mainly by methane and carbon dioxide. This mixture of gases referred to as biogas. The conversion of solids to biogas reduces dramatically the amount of solids that must be disposed in landfill. During the anaerobic treatment process, organic nitrogen compounds are converted into ammonia, sulphur compounds are converted into hydrogen sulphide, phosphorus into orthophosphates, and calcium, magnesium, and sodium are converted into a variety of salts. The end products of anaerobic digestion are methane usable for energy production, heat released during the anaerobic digestion, a nutrient rich organic slurry and other marketable inorganic products. The effluent containing particulate and soluble, organic and inorganic materials may be separated into its particulate and soluble constituents. The rate and efficiency of the anaerobic digestion process are controlled by [Lens et al 2005]. Temperature: it is an important factor for microbial activity. There are three different temperature intervals optimal for AD; these intervals are called psychrophilic (0-20°C), mesophilic (15-45°C) and thermophilic (45-75°C); Toxicants; pH and alkalinity: to maintain stable methanogenic activity, a pH between seven and eight is desired; Nutrients; Water content: it is essential for biological activity, since nutrients must be dissolved in water before they can be assimilated; in addition water enhances the mobility of microorganisms facilitating their contact with the substrate. Anaerobic digestion takes place along three different stages for waste degradation and biogas production. Hydrolysis and acidogenesis: Anaerobic bacteria degrade complex organic molecules (proteins, cellulose, lignin, and lipids) into soluble monomer molecules such as amino acids, glucose, fatty acids, and glycerol. The monomers are directly available to the next group of bacteria. The hydrolytic phase is relatively slow and may be the limiting step in the anaerobic digestion of waste such as raw cellulolytic wastes, which contain lignin. After acidogenic bacteria convert sugars, amino acids and fatty acids into organic acids (e.g., acetic, propionic, formic, lactic, butyric, or succinic acids), alcohols and ketones (e.g., ethanol, methanol, glycerol, acetone), acetate, CO2 and H2. Acetate is the main product of carbohydrate fermentation. The generated products vary with the type of bacteria as well as with culture conditions (temperature, pH, redox potential). Acetogenesis: Acetogenic bacteria convert fatty acids (e.g., propionic acid, butyric acid) and alcohols into acetate, hydrogen, and carbon dioxide which are

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used in next phase. Acetate formation is reduced in anaerobic environmental. Methanogenesis: Methanogenic bacteria are anaerobic and transform the organic acid and hydrogen into simpler end products, mainly methane and carbon dioxide. About two thirds of methane is derived from acetate conversion by methanogenic bacteria. The other third is the result of carbon dioxide reduction by hydrogen. There are different types of anaerobic digesters [Lens et al 2005]: 1. Wet fermentation systems for treatment of slurries: the Continuous Stirred Tank Reactor (CSTR) is the most used reactor type to digest low solid waste streams and is used for the anaerobic stabilization of sewage sludge produced in wastewater treatment plants and for the treatment of piggery waste. These digesters consist of a void recipient stirred by biogas recirculation or mechanical means. 2. Dry fermentation systems for the treatment of solid waste: They are smaller in size, require less process water and require less heating in relation to wet fermentation system. The dry fermentation system are plug-flow reactors. On the contrary, complete mix reactors are usually used in wet system. These digesters consist of a long tube, which enables hydrolysis and methanogenesis to be spatially separated. 3. High-rate systems for the treatment of wastewater: In contrast to slurries, organic matter in wastewater is highly diluted. These reactors are divided into two categories: systems with a suspended bacterial mass and systems with fixed bacterial films on solid surfaces. Among the first type, the contact digester and the UASB may be listed (elencados). The contact digester comprises three units: a CSTR digester, a degasser and a settler. The case study, Marsciano’s plant, uses two primary ADs, both CSTR connected with others parts of the system as described in the following. 2.2 System description The case study is a farm in Marsciano (Perugia, Italy) where pig manure is treated in two anaerobic digester to produce biogas which will be fed into a MCFC system. It serves different customers in a territory of 36 km2. As shown in figure 1, this plant is composed of: • waste adduction section; • sewage storage: that minimize the fluctuation of incoming sewage; • two groups of pumps, used for sending the sewage to the digesters; • two primary anaerobic digesters, that can work in series or in parallel configuration; • one secondary anaerobic digester working for sedimentation and for collection of biogas; • sludge dehydration system (centrifuge); • cleaning treatment of biogas (desulphurisation); • aerobic stabilization; • compost system;

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two internal combustion engine (ICE: 374 kW and 511 kW).

Fig. 1: Overview anaerobic digestion process [3]

The energy recovery section is currently represented by two ICEs with nominal power of 370 kW and 510 kW. The aim of the present study is to demonstrate the feasibility to replace ICEs with MCFC system. After the adduction section, there is an equalizer system to avoid flow fluctuations of incoming sewage. Sewage is then sent through two groups of pumps to the anaerobic digesters, two primary and one secondary. The two primary digesters may work in series or in parallel. The secondary anaerobic digester is used to increase the efficiency of biogas production and also acts as biogas storage system. The plant has the sludge treatment line and the biogas treatment line (desulphurisation of biogas). 2.3 Use of Molten Carbonate Fuel Cell System This work analyzes the use of MCFC system instead of ICE system. An internal report of Marsciano’s plant [Lunghi et al 2001] shows that (in 2003) 2,16 million m3 of biogas (chemical composition shown in table 1) have been produced. Taking into account the Low Heating Value (LHV) of biogas and the electric efficiency of the MCFC as 50%, the potential production of electrical power is 710 kWe. Today, for ICE with an electric efficiency of 30 %, the installed electrical power in the plant is 885 kWe. MCFC system maximum power output can be calculated through the following relations:

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where: B = Volumetric flow of biogas [m3/h]; C = CH4 in biogas composition [%]; L = LHV, Low Heating Value of CH4 [kJ/m3]; E = Electric Efficiency [%]; W = Maximum Power [kW]. The values of these parameters used in this case study are shown in table 1:

Table 1: Values used in case study

3 Results and discussions Experimental analysis The chemical composition of the biogas from Marsciano plant after passing through a reforming process has been simulated in the lab. This composition is supplied and controlled by using a software. The chemical composition after reforming is shown in table 2.

Table 2: Chemical composition after reforming

The tests have been executed on a single MCFC produced by Ansaldo Fuel Cells Spa. The test rig is composed of different parts as shown in figure 2.

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Fig. 2: Outtline of the laboratory: l 1)Gas bottlees box, 2) Diigital mass fflow meters, 3) Hu umidification n system, 4) Heaters, 5) Single fuel cell, c 6) Hotp plate (temperrature 650 0C C), 7)Pressure 4 baar, 8) Electricc load, 9) Saffety system, 10) 1 Control sy ystem.

ntaining diffferent gas bottle, b is co onnected to the lab. In nlet An externaal box, con gaas composittions are con ntrolled by digital mass flow meteers. The humiidification system iss fundamen ntal to av void carboon deposition (fformation off solid carbon that couuld damage the FC). Tw wo hotplatees increase the t gaas temperatuure, while the t pressuree system co ontrols the pressure. p Thhe current for f thhe polarizattion of FC is supplieed by an electric e load d. The safeety system is coomposed off sensors to o detect leakkages of H2, CO2 and others danngerous gases. Thhe control system s consists of a ppc to control every partt of the sysstem and tests reesults. The first f test gen nerates the ppolarization n curve, sho owing the vaariation of the t fuuel cell vooltage in reelation to the curren nt density (b) for fixxed inlet gas g coompositionss. This polarrization currve is shown n in figure 3. 3

Fig. 33: Polarizatio on Curve

The secondd test shows the voltagge as a functtion of curreent density for a range of fuuel utilizatioon (c), show wn in figure 4.

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Fig. 4: Voltage-Current-Uf

These curves show that biogas with these compositions is suitable for feeding fuel cells, after passing through clean-up process.

where: e- = electron flow [mole/s] H2 = inlet hydrogen flow [mole/s] CO = inlet carbon monoxide flow [mole/s] The aim of experimental tests was to assess the possibility of using this alternative gas under different values of relevant input parameters i.e. total current, current density, utilization factor. Economical analysis The essential part of this work is the economical analysis described as the relation between the current use of ICE and the use of MCFC. This relationship has been established using NPV (Net Present Value, (d)) under two different scenarios, with and without economic incentives, with a sensitivity analysis assuming three different initial investments,1500€/kW, 2000€/kW, 3000€/kW. Moreover, the relative importance of the cogeneration in both cases has been studied. After performing the technical analysis, the economical analysis using as term of comparison NPV is defined as:

with: I0 = investment cost [€], n = years.


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where: i = discount rate, N = Cash Flow [€/year], ACF = Discounted Cash Flow [€/year], The cash flow is the difference between the benefits (through the sale of the thermal and electrical power) and total cost (P + M):

R = Benefits [€/year], P = Personal cost [€/year], M = Maintenance cost [€/year], r = inflation rate. Maintenance and personal costs aren’t constants in the time, but in case study they are considered in relation of inflation and discount rates. In the case study the relation between the use of MCFC and present scenario (two ICEs) has been analysed. In both cases, the possibility of co-generation has been considered. The goal of this analysis is highlighting the most relevant parameters for the insertion of MCFC in fuel cell’s market. A software for macro and micro economical laws has been used.

Table 3: Economic real data

The economic real data shown in table 3 has been used. Figure 5 shows the evolution of NPV with time for both scenarios: future use of MCFC and present use of ICE. In this figure, it is shown that the payback-time of MCFC system with initial investment of 1500 €/kW and 2000 €/kW is about one year, as ICE present system; the payback-time of MCFC system for initial investment of 3000 €/kW is about two years.

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Fig. 5: NPV with incentives and with co-generation

The slope of MCFC systems curves is the same or higher than the slope for ICE system curve. After the cross point of these curves, the higher slope of MCFC curves, the higher economical benefits for higher economical efficiency. In figure 6, the case without economical incentives is considered and it shows that the payback-time of CFC system with initial investment of 1500 €/kW and ICE system is the same, about two years; for MCFC system with initial investment of 2000 €/kW is four years and with initial investment of 3000 €/kW is twelve years. In figure 7, the case without co-generation and with economical incentives is considered showing the importance of cogeneration with different payback-time, between one and two years for MCFC system with initial investment 1500 €/kW, 2000 €/kW and for ICE system and of three years for initial investment of 3000 €/kW.

Fig. 6: NPV without incentives and with co-generation


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Fig. 7: NPV with incentives and without co-generation

The case without co-generation and without economic incentives is not interesting because it has higher payback-times. It is shown in figure 8.

Fig. 8: NPV without incentives and without co-generation

All these curves show that, with initial investment of 1500 €/kW, 2000 €/kW and 3000€/kW, the use of MCFC is more economic than the ICE currently installed ICE. Moreover, economic incentives are very important, without them both configurations would have less advantages. Lastly, co-generation is essential for the economical and technical efficiency in both scenarios: use of ICE and use of MCFC system.

4 Conclusion The conclusion of the study is that, although fuel cell systems diffusion is still limited due to the high manufacturing costs and the need of technology improvements, the possibility of integrating an anaerobic digester with an MCFC represents a potential business especially if the environmental benefits are translated into an economical saving.

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The tests carried out by Ansaldo dealing with single fuel cells, have been mainly focused on the use of biogas with low LHV (Low Heating Value) as biogas by anaerobic digestion. Their eventual use in MCFC involves a reduction of the emissions in atmosphere and higher electrical efficiency. In the last phase of the work, the economic analysis of the real case of Marsciano (Perugia) comparing new technologies (MCFC) to traditional ones (ICE) has been performed. This analysis, performed by using an Excel application, provides a number of diagrams that show the future possible use of fuel cell for thermal and electrical power production.

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Received: January 18, 2014