Thermodynamic analysis of solid oxide fuel cell system using different ethanol reforming processes

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Thermodynamic analysis of solid oxide fuel cell system using different ethanol reforming processes Chollaphan Thanomjit a, Yaneeporn Patcharavorachot b, Pimporn Ponpesh a, Amornchai Arpornwichanop a,* a

Computational Process Engineering Research Unit, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand b School of Chemical Engineering, Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok 10520, Thailand

article info

abstract

Article history:

In this study, a performance of a solid oxide fuel cell (SOFC) system integrated with

Received 19 July 2014

different ethanol reforming processes (i.e., steam reforming (SR), partial oxidation (POX)

Received in revised form

and autothermal reforming (ATR)) is investigated with the aim to determine a suitable

25 March 2015

ethanol reforming process for the SOFC system. The thermodynamic analysis of the SOFC

Accepted 27 March 2015

system operated under steady state conditions was performed using flowsheet simulator.

Available online 23 April 2015

A detailed electrochemical model incorporating all voltage losses (i.e., activation, ohmic and concentration losses) was considered. The simulation results showed that increases in

Keywords:

reformer and SOFC temperatures can improve the electrical performance of the SOFC

Solid oxide fuel cell system

system. The electrical performance of the SOFC-SR is maximized because this reforming

Ethanol

process provides the highest hydrogen yield. However, because the SOFC included an in-

Steam reforming

ternal methane reformation, electrical performances of SOFC systems with different

Partial oxidation

reforming systems are slightly different. When the thermal efficiency was determined, it

Autothermal reforming

was revealed that the SOFC-POX system had a higher thermal efficiency with an increasing

Thermodynamic analysis

O/E and decreasing reformer and SOFC temperatures. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The combustion of finite, nonrenewable fossil fuels (e.g., coal, oil, and natural gas) to provide electrical energy and the increased energy demands have led to energy shortages and environmental problems, which include global warming due

to the emission of greenhouse gases and air pollutants. Many countries are pursuing efforts to develop sustainable and ecofriendly technologies for power generation to replace conventional combustion heat engines. One of these alternatives, the solid oxide fuel cell (SOFC), is an electrochemical conversion device that directly produces electrical energy from the chemical energy of a fuel and has received much attention.

* Corresponding author. Tel.: þ66 2 218 6878; fax: þ66 2 218 6877. E-mail address: [email protected] (A. Arpornwichanop). http://dx.doi.org/10.1016/j.ijhydene.2015.03.155 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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SOFC operated at high temperatures do not require expensive precious metal catalysts like low temperature fuel cells. In addition, a good integration of the high temperature SOFC with a fuel processor that requires a certain high-temperature external heat results in high system efficiency [1]. Ethanol has been considered as a fuel in fuel cell applications due to its potential for being made in a renewable manner. In addition, ethanol has attractive attributes, such as storage ease and increased handling and distribution safety, because of its lower toxicity and volatility [2,3]. Although there are many studies focusing on the direct operation of SOFCs with ethanol [4e8], it was found that ethanol causes the degradation of conventional anodes (Ni-YSZ) by carbon deposition. Consequently, ethanol should be at least partially reformed into synthesis gas that contains hydrogen and carbon monoxide (in addition to carbon dioxide and water) before feeding into the SOFC. There are three major methods for producing hydrogen from ethanol: steam reformation (SR), partial oxidation (POX) and autothermal reformation (ATR). Each process can be accomplished under different operating temperatures, which leads to differences in hydrogen yield and energy consumption [9e11]. These factors strongly affect the performance of a SOFC; therefore, the selection of the ethanol reforming process is an important task. Recently, a number of studies have focused on identifying favorable operating conditions at which the maximum H2 is produced [9,12e14]. There are few studies that have considered SOFC systems integrated with ethanol reforming processes [1,15e17]. Srisiriwat [15] presented a system combining a SOFC and autothermal reforming and studied the effect of a water-gas shift (WGS) reactor on the system efficiency. Hong et al. [16] studied the effects of a reforming method on the electric efficiency and heat efficiency of the ethanol-fueled SOFC system. They reported that although the SOFC based on the auto-thermal reforming process has a lower electric efficiency than that with the steam reforming process, its energy efficiency can be improved. From the literature, it can be seen that no other thermodynamic analyses of SOFC systems based on different ethanol reforming processes have been explored. In this work, the performance of a SOFC system is investigated by considering both the electrical and thermal performances. The aim is to determine a suitable ethanol reforming process for the SOFC system. A thermodynamic analysis of the SOFC system integrated with different reforming processes (i.e., steam reforming, partial oxidation and autothermal reforming) is performed using flowsheet simulator. The SOFC stack model is first developed by using existing AspenPlus functions and unit operation models with minimum requirements for linking subroutines; it provides a convenient way to perform detailed thermodynamic and parametric analyses of SOFC systems and can be easily extended to study the entire plant process [18]. Then, effects of key operating parameters in a SOFC system, such as the SOFC temperature, reforming temperature, steam-to-ethanol ratio, and oxygen-to-ethanol ratio, on hydrogen production and electrical efficiency are studied. Additionally, simulation results are compared to identify the favorable operating condition of each reforming process.

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SOFC system Process flow diagram A schematic of the SOFC system integrated with an ethanol reforming process is shown in Fig. 1. It consists of three main parts: (1) an ethanol reforming process in which ethanol is converted into a hydrogen-rich gas, (2) a SOFC that subsequently generates electricity and (3) an afterburner where the residual fuel from SOFC is combusted in order to supply heat to other parts of the SOFC system. The ethanol and reforming agents (i.e., steam, air and combined air/steam) are fed into the system at ambient conditions. The models of the ethanol reforming process, SOFC and afterburner are discussed separately.

Ethanol reforming process In the ethanol reforming process, ethanol is converted into a gaseous mixture of H2O, CO, CO2, CH4, unreacted EtOH and the main species, H2. Other hydrocarbon compounds, such as acetaldehyde and ethylene are considered intermediate products, which are quickly converted to simpler molecules at high contact times and temperatures (>673 K) [19]. A reforming process is generally composed of a preheater and a reformer. The ethanol and reforming agents (steam, air and combined air/steam [20]) are vaporized and heated before entering a reformer (represented in AspenPlus by the reactor module labeled “REFORMER”), which uses the equilibrium reactor module type RGibbs. The reformer is assumed to operate under isothermal and equilibrium conditions. The direct minimization of the Gibbs free energy solved by the Lagrange's undetermined method is used to compute the equilibrium composition of synthesis gas. In general, the main reactions that occur in each ethanol reformer are steam reforming, partial oxidation. Additionally, there are four possible side reactions, dependent on the type of reforming processes, namely, methane steam reforming, methane oxidation, water gas shift reaction and methanation [9,21]. In this study, products of ethanol reforming are predicted based on a thermodynamic analysis without considering the effects of catalysts. There are a number of other studies that have investigated the synthesis and use of catalysts for ethanol reforming [22e25].

Solid oxide fuel cell The hydrogen required for a SOFC operation is produced from ethanol in the unit represented in AspenPlus as “REFORMER”. The simulation of the SOFC unit is separated into two parts: the first is the “ANODE” in which electrochemical reactions occur inside the cell, and the second is the “CATHODE”, which supplies oxygen as an oxidant to the “ANODE” as represented by the separator module, Sep. A SOFC operates at a high temperature between 600 and 1000  C. Therefore, the direct oxidation in the SOFC of the CO and CH4 contained in the reformate gas is feasible without a catalyst, but is less favored than the water gas shift reaction of CO to H2 and the reformation of CH4 to H2 [26]. In this study, however, hydrogen was

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Fig. 1 e Schematic of the SOFC system integrated with ethanol reforming process. assumed to be the only electrochemically reacting fuel because of the more readily oxidized fuel. The “ANODE” block is represented by the equilibrium reactor module, RGibbs, where methane steam reforming, water gas-shift reaction and electrochemical reaction are assumed to be at chemical equilibrium at given temperatures [18]. The hydrogen produced at the anode from the internal steam methane reformation and the water gas shift reaction participates in the electrochemical reaction (H2 þ 0.5O2 –> H2O). Hydrogen is consumed by the oxidation reaction at the anode and releases electrons to the external circuit. Simultaneously, oxygen in the air accepts electrons from the external circuit, is reduced into oxygen ions at the cathode and reacts with hydrogen through electrolyte to form water and electrons at the anode. The direct-current electricity is generated by electron flow from the anode to the cathode. To investigate the performance of an integrated SOFC system and reforming process, a generalized steady-state model is considered [19,27]. The electrochemical equations relate to the fuel and air compositions, cell temperature, pressure, current density and cell parameters, such as the active area and the electrode thickness. These equations will be described as follows: The theoretical open-circuit voltage (EOCV) is the difference between the thermodynamic potentials of the electrode reactions and can be expressed by the Nernst equation: OCV

E

0 1 1
(1)

where E0 is the reversible open-circuit voltage at standard pressure and is a function of the operating temperature: E0 ¼ 1:253  2:4516  104 T

(2)

However, the actual voltage (V) is always less than the open-circuit voltage due to internal resistance and overpotential losses, which include activation, ohmic and concentration overpotentials:   V ¼ EOCV  hact;anode þ hact;cathode þ hohm þ hconc;anode þ hconc;cathode (3) where hohm is the ohmic loss, hact,anode and hact,cathode are the activation overpotentials, and hconc,anode and hconc,cathode

represent the concentration overpotentials at the anode and the cathode, respectively. The activation overpotential is related to the electrode kinetics at the reaction site and can be expressed by the nonlinear ButlereVolmer equation:      anF ð1  aÞnF hact;electrode  exp  hact;electrode i ¼ i0;electrode exp
2 3 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2
(5)

electrode 2 fanode; cathodeg The exchange current density (i0,electrode) is dependent on the operating temperature, as shown in Eq. (6). The preexponential factor and the activation energy of the anode and cathode are used for the calculation of i0,electrode, given by Aguiar et al. [29] and shown in Table 1: i0;electrode ¼

 
(6)

electrode 2 fanode; cathodeg The ohmic overpotential is caused by the resistance along the flow of ions through the electrolyte, the flow of electrons through the electrodes and current collectors, and by the contact resistance between cell components. This loss obeys Ohm's law:

Table 1 e Pre-exponential factor and activation energy. Anode kanode Eanode

Cathode 6.54  1011 U1 m2 140 kJ mol1

kcathode Ecathode

2.35  1011 U1 m2 137 kJ mol1

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nOhm ¼ iROhm

(7)

where i is the current density, ROhm is the internal electrical resistance calculated from the conductivity of the individual layers (assuming negligible contact resistances, cross-plane charge flow, and resistance from series connections) as given by: ROhm ¼

tanode telectrolyte tcathode þ þ ; sanode selectrolyte scathode

nconc

!

!

pH2 O;TPB ¼ pH2 O;f þ

pO2 ;TPB


1 0


Hydrogen combustion :

H2 þ 0:5O2 / H2 O;

Carbon monoxide combustion :

CO þ 0:5O2 / CO2 :

(13) (14)

Solution approach

(9)

where the first term on the right-hand side refers to the anodic concentration overpotential (hconc,anode), and the second term refers to the cathodic concentration overpotential (hconc,cathode). pH2O,TPB, pH2,TPB, and pO2,TPB, are the partial pressures of H2, H2O, and O2 at the three-phase boundaries, respectively. Their diffusion transport in a porous electrode can be described by Fick's model: pH2 ;TPB ¼ pH2 ;f 

assumed to reach complete combustion (i.e., 100% conversion). The combustion reactions occurring in afterburner are as follows:

(8)

where tanode, telectrolyte and tcathode represent the thickness of the anode, electrolyte, and cathode layers, respectively, sanode and scathode are the electronic conductivity of the anode and cathode, respectively, and selectrolyte is the ionic conductivity of the electrolyte. The concentration overpotential is the voltage loss associated with the transport of gaseous reactants through porous electrodes. It can be determined from the difference in the open-circuit voltage based on the reactant and product concentrations at three-phase boundaries (TPB) and bulk concentrations. The concentration overpotential (nconc) is determined as:
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(10)

(11)

(12)

Although the gas-diffusion coefficient (Deff;i ) increases with increasing temperature, the effect of an increased diffusion rate on the concentration overpotential is less pronounced [28]. Therefore, the gas-diffusion coefficient is assumed constant in this study.

Afterburner After undergoing the electrochemical and chemical reactions in the SOFC, the unreacted fuel gases from the anode enter the afterburner for the combustion of the remaining hydrogen and carbon monoxide (labeled as the “EX-ANODE” stream in AspenPlus) with the excess oxygen from the cathode (“EXCATHODE” stream). The exhaust gas from afterburner is at a sufficiently high temperature that the heat can be thermally utilized for other sections of the SOFC system. An afterburner model is represented by the reactor module RStoic and set at adiabatic conditions. The reactions specified in this block are

AspenPlus is used to model the performance of a SOFC system integrated with different ethanol reformation processes (i.e., SR, POX and ATR). This combined system is based on a steady-state, detailed electrochemical balance that accounts for all voltage losses (i.e., activation, ohmic and concentration losses). The values of the physical parameters of the cell components are listed in Table 2. Standard operating conditions and operating temperatures for each unit in the system are listed in Tables 3 and 4, respectively. The minimization of the Gibbs free energy is used to determine equilibrium compositions. The equation of state used in the calculation is based on the Peng-Robinson (PEN-ROB) method which is used to predict the behavior of hydrocarbons and light gases [30e32]. The gas mixtures compositions contained C2H5OH, H2O, H2, O2, N2, CH4, CO and CO2. C2H4 and C2H4O can be considered intermediates of an incomplete reformation reaction and are not thermodynamically stable products [9]. Further, no carbon formation is assumed to occur under this operating condition. When all operating conditions and physical parameters are specified in the flowsheet simulator, the gas composition obtained from the reformer can be determined. Synthesis gas, containing mainly H2, further reacts with oxygen to produce the direct-current electricity in the SOFC stack. Other than the gas composition, the flowsheet simulator can also determine the heat duty of all unit operations in the SOFC system from energy balance equations. Based on the electrochemical model of SOFC (Eqs. (1)e(12)), the performance of the SOFC system can be quantified from the hydrogen yield, electrical efficiency (hSOFC) and thermal efficiency (hThermal), which are defined as follows:

Table 2 e Physical parameters of cell components [2]. Anode thickness, tanode Cathode thickness, tcathode Electrolyte thickness, telectrolyte Anode electrical conductivity, sanode Cathode electrical conductivity, scathode Electrolyte ionic conductivity, selectrolyte Anode diffusion coefficient, Deff,anode Cathode diffusion coefficient, Deff,cathode Active cell surface, A

500 mm 50 mm 20 mm 80  103 U1 m1 8.4  103 U1 m1 33.4  103 exp(-10,300/T) U1 m1 33.4  105 m2 s1 1.37  105 m2 s1 100 m2

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Table 3 e Standard operating conditions.

Ethanol molar feed flow rate (l/min) Water molar feed flow rate (l/min) Air molar feed flow rate (l/min) Reforming  temperature ( C) Stack exhaust  temperature ( C) Fuel utilization (%) Voltage (V) Current density (mA/cm2) Gross AC efficiency (LHV) (%)

Hydrogen yield ¼

hSOFC ¼

SOFC-SR

SOFC-POX

SOFC-ATR

0.962

0.962

0.962

0.604

e

0.604

e

970.3

970.3

700

700

700

800

800

800

85 0.834 273.39

85 0.855 227.82

85 0.839 227.82

66.71

57.23

55.93

Fig. 2 e Comparison of cell characteristics between simulation results and experimental data [33].

FH2 ; FEtOH;in

(15)

PSOFC  100; FEtOH;in $LHVEtOH

hThermal ¼

(16)

Qproduced  Qconsumed  100; FEtOH;in $LHVEtOH

(17)

where FH2, FEtOH,in, PSOFC and LHVEtOH are hydrogen molar flow rate, ethanol inlet molar flow rate, the power and lower heating value of ethanol (1230 kJ/mol of ethanol), respectively. In addition, Qproduced is the energy content of the exhaust gas converted to low temperature (200  C) and Qconsumed, the consumed energy, is the heat required for ethanol and water vaporizations, ethanol and steam pre-heating, and the reforming process.

Results and discussion To ensure that the electrochemical model can reliably predict the SOFC performance, this model was validated with the experimental data of Zhao and Virkar [33]. In their experiment, the inlet fuel consisted of 97% H2 and 3% H2O, and the inlet oxidant comprised 21% O2. The thickness of the anode, cathode, and electrolyte were 1000, 20 and 8 mm, respectively. Based on the same operating conditions as reported in Refs. [33], the currentevoltage curve of the SOFC was characterized at temperatures of 1023 K, 1073 K, and 1123 K under 1 bar

pressure. The comparison of our model predictions and the experimental data in terms of cell voltage at different current densities and operating temperatures is shown in Fig. 2. It can be seen that the model prediction shows good agreement with the experimental data in the literature. In this section, the impacts of important operating parameters (i.e., steam-to-ethanol ratio, oxygen-to-ethanol ratio, reformer temperature and cell temperature) are investigated in terms of the hydrogen yield, and thermal and electrical efficiencies. It is expected that each reforming process can provide favorable operating conditions for the SOFC system.

Effect of steam and oxygen-to-ethanol ratio At standard conditions, the inlet molar flow rate of ethanol is 1 kmol/h, the steam-to-ethanol ratio (S/E) is 2 and/or the oxygen-to-ethanol ratio (O/E) is 0.5. Fig. 3 presents the effects of S/E and O/E on hydrogen yields from the three reforming processes. It can be seen that an increasing S/E more significantly increases the hydrogen yield in the SOFC-SR process. This is because the steam reforming and water gas-shift reactions are in thermodynamic equilibria. Likewise, increasing the O/E increases the hydrogen yield in the SOFC-POX process

Table 4 e Operating temperatures for each unit in the system. Unit Vaporizer (HEATER1 and HEATER2) Preheater Reformer Cell stack Cooler Preheater air to cathode (HEATER3)



Temperature ( C) 100 400 700 800 200 800

Fig. 3 e Effect of steam and oxygen-to-ethanol ratio on the hydrogen yield.

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at a reformer temperature of 700  C. Although increasing the oxygen content can improve the oxidation reaction (Eq. (2)), the additional air dilutes the H2 concentration. For the SOFCATR process, the increase in S/E (keeping O/E constant) indicates that the SR reaction in the reformer increases and thus, the H2 yield can be achieved. However, the increase in O/ E (keeping S/E constant) in the SOFC-ATR process results in the opposite trend as that of the SOFC-POX process. From the simulation results, it can be seen that the hydrogen yield is reduced at higher O/E. Figs. 4 and 5 present the electrical and thermal efficiencies as a function of S/E and O/E. As seen in Fig. 4, increases in not only S/E but also O/E cause decreases in the electrical efficiency of all systems. Although increased S/E and O/E can result in higher H2 yields, the higher amount of steam and/or oxygen dilutes the H2 concentration in the reformer effluent. When the H2 concentration decreases, the electrochemical reaction is reduced. This leads to reductions in the cell voltage and electrical efficiency. The effects of S/E and O/E on the thermal efficiency are illustrated in Fig. 5. It can be seen that the thermal efficiency decreases with an increasing S/E in both the SOFC-SR and SOFC-ATR systems. This is because raising the S/E increases the water content in the system which in turn increases the vaporizer and preheater heat duties and the reformer duty. Increasing O/E increases the thermal efficiencies in both the SOFC-POX and SOFC-ATR systems. Although the heat duty of the air preheater is increased in a system with higher oxygen content, the reformer duty is decreased because the higher oxygen content promotes the exothermic oxidation reaction and leads to a higher energy production.

Effect of reformer temperature In this section, the impact of reformer temperature on hydrogen yield, electrical efficiency and thermal efficiency are investigated, as illustrated in Figs. 6e8, respectively. In Fig. 6, it can be seen that when the reforming temperature is increased from 400  C to 700  C, the hydrogen yield increases with increasing reformer temperature in all systems. This is because the rate of all chemical reactions is more pronounced

Fig. 4 e Effect of steam and oxygen-to-ethanol ratio on the Electrical efficiency.

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Fig. 5 e Effect of steam and oxygen-to-ethanol ratio on the thermal efficiency.

Fig. 6 e Effect of reformer temperature on the hydrogen yield.

at the higher temperature. However, the hydrogen yield is slightly reduced when the reformer temperature is increased to 800e1000  C. This is mainly due to an unfavorable wateregas-shift reaction. From the simulation results, it can be observed that for a reformer temperature range of 400e700  C, the SOFC-ATR system can provide a higher hydrogen yield compared with the SOFC-SR system. However, when the reformer temperature is greater than 800  C, the oxidation reaction is suppressed. As a result, the hydrogen yield of the

Fig. 7 e Effect of reformer temperature on the electrical efficiency.

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Fig. 8 e Effect of reformer temperature on the thermal efficiency.

SOFC-ATR system is less than that of the SOFC-SR system. From Fig. 6, it can be concluded that the SOFC-SR system can provide the highest hydrogen yield, followed by the SOFC-ATR and the SOFC-POX systems, respectively. Fig. 7 shows the influence of the reformer temperature on the electrical efficiency. It is found that the electrical efficiency can be improved when the reformer temperature increases from 400 to 700  C. This is because the higher H2 concentration in the reformer effluent increases the cell voltage and thereby increases the electrical efficiency. The SOFC-SR system has the highest electrical efficiency compared with other systems because the hydrogen yield of the SOFC-SR system is the highest. The SOFC-POX system has the highest thermal efficiency, followed by the SOFC-ATR and the SOFC-SR systems, as shown in Fig. 8. This is because the SOFC-POX system is exothermic and thus, can produce more heat than the other systems. For the individual reforming process, it is found that the thermal efficiency decreases with increasing reformer temperature. A higher reformer temperature results in a higher energy requirement for reformer duty. Subsequently, the thermal efficiencies of all systems are reduced.

Effect of SOFC temperature Figs. 9 and 10 present the effect of the SOFC temperature on the electrical and thermal efficiencies in the three reforming

Fig. 9 e Effect of SOFC temperature on the electrical efficiency.

Fig. 10 e Effect of SOFC temperature on the thermal efficiency.

processes. Fig. 9 shows that an increased SOFC temperature can enhance the electrical efficiency because the electrochemical reaction rate is increased and voltage losses can be reduced, resulting in an increase in the cell voltage. However, because the internal reforming of methane in the SOFC is also considered, the electrical performances of SOFC systems with different reforming systems are slightly different. As shown in Fig. 10, the effect of the SOFC temperature on the thermal efficiency is observed to have the opposite trend as that with electrical efficiency. Increasing the SOFC temperature leads to a reduction in the thermal efficiency. As an endothermic reaction, because methane steam reforming is more pronounced in the SOFC, the SOFC operates at a cooler temperature, resulting in reduced heat generation from the SOFC. Therefore, less air is required to maintain the cell operating temperature, which positively affects the heat duty of the afterburner.

Conclusions A performance analysis of SOFC system integrated with different ethanol reforming processes (i.e., steam reforming, partial oxidation and autothermal reforming) was conducted and compared by considering their electrical and thermal performances. From the simulation results, it was found that each reforming process can provide unique and favorable operating conditions for a SOFC system. In decreasing order of hydrogen concentration in the reformate are ranked SR > ATR > POX. Additionally, wider ranges of S/E, O/ E and reformer temperatures were permitted. This strongly influences the electrical efficiency of the SOFC system when the SOFC temperature is kept constant at 800  C. However, the SOFC-SR system requires the greatest energy consumption, and thus, the lowest thermal efficiency is observed in this system without considering the internal reformation and synergistic exchange of heat and steam. For the SOFC-POX and SOFC-ATR systems, it can be seen that their electrical efficiencies are the same, but the thermal efficiency is much higher in the SOFC-POX system. Nevertheless, the addition of air in the SOFC-POX and SOFCATR systems should be carefully considered. This is because the hydrogen concentration in the reformer effluent is

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diluted by nitrogen and results in a deterioration of SOFC performance. Therefore, the operating conditions of the reforming process and SOFC should be selected to compromise between the electrical and thermal efficiencies of the SOFC system.

Acknowledgments Support from the Thailand Research Fund and the Ratchadaphiseksomphot Endowment Fund (the 90th Anniversary of Chulalongkorn University Fund) is gratefully acknowledged.

Nomenclature Deff,anode effective diffusivity coefficient in the anode, m2 s1 Deff,cathode effective diffusivity coefficient in the cathode, m2 s1 OCV E open-circuit voltage, V E0 reversible open-circuit voltage at standard pressure, V Eelectrode activation energy of the exchange-current density, kJ mol1 F Faraday's constant, C mol1 Fi molar flow rate of component i, kmol h1 i current density, A m2 i0,electrode exchange-current density, A m2 kelectrode pre-exponential factor of the exchange current density, A m2 LHVEtOH lower heating value of ethanol n number of electrons participating in the electrochemical reaction P pressure, bar partial pressure of component i, bar pi partial pressure of component i at three phase pi,TPB boundary, bar power, kW PSOFC Qconsumed consumed energy, kW Qproduced produced energy, kW internal electrical resistance, U m2 ROhm < gas constant, kJ mol1 K1 T temperature, K V cell voltage, V

Greek symbols a transfer coefficient activation overpotential, V hact concentration overpotential, V hcon Ohmic overpotential, V hohm electrical efficiency, % hSOFC hThermal thermal efficiency, % anode electrical conductivity, U1 m1 sanode scathode cathode electrical conductivity, U1 m1 selectrolyte electrolyte ionic conductivity, U1 m1 tanode anode thickness, m tcathode cathode thickness, m telectrolyte electrolyte thickness, m

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references

[1] Saebea D, Patcharavorachot Y, Arpornwichanop A. Analysis of an ethanol-fuelled solid oxide fuel cell system using partial anode exhaust gas recirculation. J Power Sources 2012;208:120e30. [2] Arpornwichanop A, Chalermpanchai N, Patcharavorachot Y, Assabumrungrat S, Tade M. Performance of an anodesupported solid oxide fuel cell with direct-internal reforming of ethanol. Int J Hydrogen Energy 2009;34:7780e8. [3] Arteaga LE, Peralta LM, Kafarov V, Casas Y, Gonzales E. Bioethanol steam reforming for ecological syngas and electricity production using a fuel cell SOFC system. Chem Eng J 2008;136:256e66. [4] Assabumrungrat S, Pavarajarn V, Charojrochkul S, Laosiripojana N. Thermodynamic analysis for a solid oxide fuel cell with direct internal reforming fueled by ethanol. Chem Eng Sci 2004;59:6015e20. [5] Cimenti M, Hill JM. Thermodynamic analysis of solid oxide fuel cells operated with methanol and ethanol under direct utilization, steam reforming, dry reforming or partial oxidation conditions. J Power Sources 2009;186:377e84. [6] Diethelm S, Van herle J. Ethanol internal steam reforming in intermediate temperature solid oxide fuel cell. J Power Sources 2011;196:7355e62. [7] Leone P, Lanzini A, Ortigoza-Villalba GA, Borchiellini R. Operation of a solid oxide fuel cell under direct internal reforming of liquid fuels. Chem Eng J 2012;191:349e55. [8] Laosiripojana N, Assabumrungrat S. Catalytic steam reforming of methane methanol and ethanol over Ni/YSZ: the possible use of these fuels in internal reforming SOFC. J Power Sources 2007;163:943e51. [9] Rabenstein G, Hacker V. Hydrogen for fuel cells from ethanol by steam-reforming, partial-oxidation and combined autothermal reforming: a thermodynamic analysis. J Power Sources 2008;185:1293e304. [10] Hernandez L, Kafarov V. Use of bioethanol for sustainable electrical energy production. Int J Hydrogen Energy 2009;34:7041e50. [11] Tsiakaras P, Demin A. Thermodynamic analysis of a solid oxide fuel cell system fuelled by ethanol. J Power Sources 2001;102:210e7. [12] Srisiriwat N, Therdthianwong A, Therdthianwong S. Thermodynamic analysis of hydrogen production from ethanol in three different technologies. In: The 2nd joint international conference on “sustainable energy and environment (SEE 2006)”. Bangkok, Thailand; 21e23 November 2006. [13] Wang W, Wang Y. Thermodynamic analysis of hydrogen production via partial oxidation of ethanol. Int J Hydrogen Energy 2008;33:5035e44. [14] Lima da Silva A, Malfatti CF, Muller IL. Thermodynamic analysis of ethanol steam reforming using Gibbs energy minimization method: a detailed study of the conditions of carbon deposition. Int J Hydrogen Energy 2009;34:4321e30. [15] Srisiriwat A. High temperature solid oxide fuel cell integrated with autothermal reformer. In: 2nd IEEE international conference on power and energy (PECon 08). Johor Baharu, Malaysia; 1e3 December 2008. [16] Hong WT, Yen TH, Chung TD, Huang CN, Chen BD. Efficiency analyses of ethanol-fueled solid oxide fuel cell power system. Appl Energy 2011;88:3990e8. [17] Saebea D, Authayanun S, Patcharavorachot Y, Paengjuntuek W, Arpornwichanop A. Use of different renewable fuels in a steam reformer integrated into a solid oxide fuel cell: theoretical analysis and performance comparison. Energy 2013;51:305e13.

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[18] Zhang W, Croiset E, Douglas PL, Fowler MW, Entchev E. Simulation of a tubular solid oxide fuel cell stack using AspenPlus™ unit operation models. Energy Convers Manag 2005;46:181e96. [19] Arteaga-Perez LE, Casas Y, Peralta LM, Kafarov V, Dewulf J, Giunta P. An auto-sustainable solid oxide fuel cell system fueled by bio-ethanol process simulation and heat exchanger network synthesis. Chem Eng J 2009;150:242e51. [20] Kang I, Bae J, Bae G. Performance comparison of autothermal reforming for liquid hydrocarbons, gasoline and diesel for fuel cell applications. J Power Sources 2006;163:538e46. [21] Ioannides T. Thermodynamic analysis of ethanol processors for fuel cell applications. J Power Sources 2001;92:17e25. [22] Srisiriwat N, Therdthianwong S, Therdthianwong A. Oxidative steam reforming of ethanol over Ni/Al2O3 catalysts promoted by CeO2, ZrO2 and CeO2eZrO2. Int J Hydrogen Energy 2009;34:2224e34. [23] Cai W, Wang F, Van Veen AC, Provendier H, Mirodatos C, Shen W. Autothermal reforming of ethanol for hydrogen production over an Rh/CeO2. Catal Today 2008;138:152e6. [24] Fierro V, Akdim O, Provendier H, Mirodatos C. Ethanol oxidative steam reforming over Ni-based catalysts. J Power Sources 2005;145:659e66. [25] de Lima SM, da Cruz IO, Jacobs G, Davis BH, Mattos LV, Noronha FB. Steam reforming, partial oxidation, and oxidative steam reforming of ethanol over Pt/CeZrO2. J Catal 2008;257:356e68.

[26] EG&G Technical Services, Inc. Fuel cell handbook. 5th ed. Virginia: U.S. Department of Energy. National Technical Information Service; 2000. [27] Perna A. Hydrogen from ethanol: theoretical optimization of a PEMFC system integrated with a steam reforming processor. Int J Hydrogen Energy 2007;32:1811e9. [28] Ni M, Leung DYC, Leung MKH. A review on reforming bioethanol for hydrogen production. Int J Hydrogen Energy 2007;32:3238e47. [29] Aguiar P, Adjiman CS, Brandon NP. Anode-supported intermediate temperature direct internal reforming solid oxide fuel cell. I: model-based steady-state performance. J Power Sources 2004;138:120e36. [30] Tang H, Kitagawa K. Supercritical water gasification of biomass: thermodynamic analysis with direct Gibbs free energy minimization. Chem Eng J 2005;106:261e7. [31] Salemme L, Menna L, Simeone M. Analysis of the energy efficiency of innovative ATR-based PEM fuel cell system with hydrogen membrane separation. Int J Hydrogen Energy 2009;34:6384e92. [32] Freitas ACD, Guirardello R. Comparison of several glycerol reforming methods for hydrogen and syngas production using Gibbs energy minimization. Int J Hydrogen Energy 2014;39:17969e84. [33] Zhao F, Virkar AF. Dependence of polarization in anodesupported solid oxide fuel cells on various cell parameters. J Power Sources 2005;141:79e95.