i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 1 1 4 4 5 e1 1 4 5 7

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Thermodynamic analysis of a proton conducting

SOFC integrated system fuelled by different
renewable fuels

Dang Saebea a, Amornchai Arpornwichanop b,c,

Yaneeporn Patcharavorachot d,*
a
Research Unit of Developing Technology and Innovation of Alternative Energy for Industries, Department of
Chemical Engineering, Faculty of Engineering, Burapha University, Chonburi, 20131, Thailand
b
Center of Excellence in Process and Energy Systems Engineering, Department of Chemical Engineering, Faculty of
Engineering, Chulalongkorn University, Bangkok, 10330, Thailand
c
Bio-Circular-Green-Economy Technology & Engineering Center, Department of Chemical Engineering, Faculty of
Engineering, Chulalongkorn University, Bangkok, 10330, Thailand
d
Department of Chemical Engineering, Faculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang,
Bangkok, 10520, Thailand

highlights

Performance analysis of reformer and SOFCeHþ integrated system was presented.

The use of different renewable fuels was studied to obtain the suitable fuel.

The reformer operated at 973 K fuelled by ethanol provided the maximum H2 content.

The SOFCeHþ should be operated at 973 K and current density of 1 A/cm2.

The best performance of system can be obtained from using glycerol reformate.

article info abstract

Article history: This work proposes a power generation system consisting of steam reformer and SOFCeHþ
Received 18 February 2020 fuelled by different types of fuel, i.e., ethanol, glycerol and biogas. The performance
Received in revised form analysis of integrated system is performed based on thermodynamic calculation through
28 July 2020 Aspen Plus simulator. The total of the Gibbs free energy minimization is used to determine
Accepted 30 July 2020 product composition at equilibrium. The electrochemical model not only considers all
Available online 2 September 2020 voltage losses but also includes the effect of current leakage as a result from the electrolyte
used. Considering the operating condition of steam reformer, it is found that the gas
Keywords: product contains the highest amount of hydrogen without the carbon formation when
Solid oxide fuel cell reformer is operated at 973 K with steam to carbon ratio of 1. In addition, the simulation
Proton-conducting electrolyte results show that the SOFCeHþ operated at 973 K and 1 A/cm2 can provide a suitable
Hydrogen production compromise between system performances and exhaust gas composition. The use of
Reforming glycerol reformate has the highest cell and system efficiencies and fuel utilization
Renewable fuels compared to the others. In addition, the integrated system fuelled by glycerol can release
low CO amount whereas there is more heat provided to the surrounding. Therefore, it can
be concluded that glycerol is suitable renewable fuel for SOFCeHþ integrated system.

* Corresponding author.
E-mail address: yaneeporn.pa@kmitl.ac.th (Y. Patcharavorachot).
https://doi.org/10.1016/j.ijhydene.2020.07.264
0360-3199/© 2020 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
11446 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 1 1 4 4 5 e1 1 4 5 7

© 2020 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.

SOFCs fuelled by ammonia and revealed that the SOFCeHþ

Introduction has maximum power density lower than SOFCeO2-.
Although the SOFCeHþ is inferior to SOFCeO2- in term of
Solid oxide fuel cells or SOFCs are potential alternative the actual performance, the exploration of solid oxides with
method for power generation. Since a direct conversion of fuel higher proton conductivity have been continuously done to
and oxidant into electricity can be occurred without the develop the performance of SOFCeHþ as higher as SOFCeO2-
combustion, the SOFCs are highly efficient device with envi- as the literature proposed above.
ronmentally friendliness. Among several types of fuel cell, This is because the use of electrolyte based on a proton
SOFCs operated at high temperature around 800e1000 K offer conduction can operate in temperature range from 673 to
several benefits. Firstly, a high electrochemical reaction rate 1023 K. Our previous work [15] studied the electrical perfor-
at the electrode can be occurred without the requirement of mance of SOFCeO2- and SOFCeHþ and found that the
noble metal materials. Then, hydrocarbon fuels can be SOFCeHþ has higher electrical efficiency than SOFCeO2- when
reformed within the cell stack due to the compatibility of the operating temperature is in range of 923e983 K. This in-
operating temperature. Moreover, the high-temperature dicates that SOFCeHþ is appropriate to operate at intermedi-
exhaust gas can be further used for heat and power com- ate temperature which is in turn a wider range of material and
bined system. application.
The conventional SOFCs based on an oxygen ion- One outstanding advantage of high operating temperature
conducting electrolyte (e.g. yttria doped zirconia or YSZ) of SOFC is that hydrocarbon fuels can be used as feed instead
have been widely studied in previous publications due to its of using pure hydrogen. This is because the SOFC operating
excellent chemical stability [1e3]. Apart from this electrolyte temperature is in same range of reforming temperature and
type, the electrolytes with a proton conduction (e.g. BaCeO3 thus, a direct reforming of fuel can be carried out within SOFC.
and BaZrO3) have been focused in many researches [4e9]. The Moreover, the SOFC is a tolerance to little amount of CO;
main attractive feature of proton conducting SOFC (SOFCeHþ) therefore, the reformate gas consisting of CO is not harmful
that is better than oxygen ion conducting SOFC (SOFCeO2-) is with SOFC. However, the investigation of a direct reforming
the generation of steam at the cathode side, as seen in Fig. 1. SOFCeHþ established in our previous work [16] discovered
When there is no dilution of steam at the anode side, that the direct reforming of methane within the SOFC stack is
hydrogen concentration at anode side is higher and thus, the not appropriate for SOFCeHþ. When there is no steam for-
reversible potential calculated by Nernst equation can be mation at the anode side, the conversion of hydrocarbon fuel
improved [10]. Silva and Müller [11] performed a thermody- to hydrogen cannot be promoted. This leads to two disad-
namic analysis to study the performance of glycerol-fuelled vantages: (1) hydrogen used for electrochemical reaction may
SOFC with different electrolytes (i.e., SOFCeO2 and be lower, resulting in a lower cell performance and (2) CO
SOFCeHþ). The results showed that theoretical efficiencies of produced from reforming reaction cannot be converted into
SOFCeO2 and SOFCeHþ are in the range of 80.7e89.9% and hydrogen in which excess amount of CO may cause the car-
90.3e96.7%, respectively. However, when the irreversible los- bon formation at the anode side, according to Boudouard re-
ses are considered, it was found by many researchers that action (Eq. (1)) and the reduction of CO by H2 or reverse coke
actual cell performance of SOFCeHþ is lower than that of gasification (Eq. (2)) as below:
SOFCeO2- [12e15]. This is mainly caused by higher ohmic loss
of SOFCeHþ. Ni et al. [12] studied the performance of different

Fig. 1 e Principle of a solid oxide fuel cell based on (a) an oxygen ion-conducting electrolyte and (b) a proton-conducting
electrolyte.
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2CO 4 C þ CO2 (1) renewable fuels can be used as fuel for hydrogen production
of SOFC system. Nevertheless, biomass-based fuel, such as
CO þ H2 4 C þ H2O (2) ethanol, biogas and glycerol has been received much inter-
est because it is a sustainable and renewable fuel. Biomass
In order to avoid the problems mentioned above, the not only can be derived from agricultural products which are
external reforming of hydrocarbon fuel integrated with available in Thailand but also is theoretically claimed as
SOFCeHþ is focused in this work. Although the overall effi- carbon neutral which in turn CO2 released to atmosphere is
ciency of integrated system is deteriorated, the operation of absorbed by plant crops. Ethanol (C2H5OH) produced from a
external reformer separates with SOFC is easier to heat fermentation of agricultural products (e.g., sugar cane, po-
management and control design. The investigations of an tato and cassava) can be used in a wider range of applica-
external reformer and SOFC integrated system have been tions, including the chemical solvent, fuel additive and
done in recent years. hydrogen production. Ethanol is a nontoxic liquid at a room
Kang et al. [17] presented a performance analysis of a diesel temperature and thus, it is easier to store, handle and
reformer SOFC system. They reported that the degradation of transport [22]. Glycerol (C3H8O3) as a by-product of biodiesel
reformer performance which in turn large amount of hydro- production becomes the most promising biomass-based fuel
carbon fuel in reformate causes the coke deposition at the since it can be produced from the transesterification of
anode side of SOFC and this can deteriorate the SOFC perfor- vegetable oils/animal fats and ethanol/methanol. Since
mance. Therefore, they suggested that the operation of glycerol has very low price, the conversion of glycerol to
reformer should be maintain high fuel conversion and this higher value-added chemical substance as H2 is an attractive
could reduce hydrocarbon content in reformate. Bae et al. [18] approach [23]. Biogas containing the main mixture of CH4
proposed an integrated system consisting of external and CO2 can be derived from the anaerobic digestion of the
reformer of n-C4H10 and SOFC unit. They compared the use of residual biomass (e.g., animal waste, municipal solid waste
different reformer types: steam reformers, autothermal re- or industrial wastewater). The use of biogas for a reforming
formers and catalytic partial oxidation reformers. It was found process can produce gas product composed mainly of H2
from their results that n-C4H10 should be reformed through without the need of a humidifier [24] and reduce the emis-
steam reformer since its terminal voltage was stable over sion of greenhouse gas [25].
250 h when steam to carbon ratio is 2. They suggested that the Although, a number of researchers have extensively
choice of steam to carbon ratio should be carefully selected to focused on a power generation system consisting of an
compromise between carbon deposition and SOFC perfor- external reformer and SOFC, it can be observed that a SOFC
mance. Faro et al. [19] investigated the performance of a based on oxygen ion-conducting electrolyte has been con-
biogas tri-reforming and SOFC integrated system which is cerned. Interestingly, it has not been reported on the
promising system for small and medium stationary power investigation of the combination between an external
systems. In their work, the fixed molar ratio of CH4/CO2 was reformer and a SOFCeHþ. Since the use of different elec-
fed into the tri-reforming process operated at 800  C whereas trolyte types causes a difference in operation and thus, the
the molar ratios of O2/CH4 and H2O/CH4 molar ratios are performance analysis of external reformer and SOFCeHþ
examined. The experimental results indicated that the refor- integrated system is main purpose in this work. The per-
mate stream containing CO, CH4 and CO2 could be fed into the formance of steam reformer and SOFCeHþ integrated sys-
SOFC stack without the addition of steam. Thanomjit et al. [20] tem is analysed with using various renewable fuels, i.e.,
analysed a performance of a SOFC integrated with different ethanol, glycerol and biogas. Next, the influences of oper-
ethanol reforming processes: steam reforming, partial oxida- ating conditions in reformer and SOFCeHþ are examined.
tion and autothermal reforming. They aimed to identify a Both favorable operating condition and suitable renewable
suitable ethanol reforming process for the SOFC system. As fuel for integrated system are also determined.
expectation, the SOFC integrated with steam reformer can An electrochemical model as an essential tool is used to
provide the highest electrical performance since it can pro- predict the relationship between current density and cell
duce the highest hydrogen yield compared with the others. voltage or power density of SOFCeHþ. In general, the elec-
However, this system was the lowest thermal efficiency. In trochemical model which is taken into account three voltage
contrast to the SOFC integrated with partial oxidation losses, i.e., activation, ohmic and concentration losses en-
reformer, although the electrical efficiency is deteriorated, the ables provide an electrical performance of SOFCeHþ. How-
thermal efficiency was high. In 2018, our publication [21] ever, it was reported in many publications that some
presented an integrated system of glycerol supercritical water proton-conducting electrolyte (e.g. BZCY) exhibits the
reforming and pressurized SOFC. Our work aimed to compare mixed protonic and electronic conductivity [26,27]. The
the performance of integrated system with different CO2 mixed conduction is a result of the addition of sintering aid,
adsorption processes that include the removal of CO2 simul- e.g. NiO. Li et al. [27] reported that higher addition of NiO on
taneously with reforming reaction (in situ process) and CO2 the electrolyte layer causes higher electronic conductivity.
captured after the completion of reforming process (ex situ When the electrolyte exhibits some mixed protonic and
process). The simulation results revealed that the integrated electronic conductivity, the current leakage can be occurred,
process with in situ CO2 removal process has higher electrical and this results in a lowered cell performance. Therefore, in
efficiency than the other. this work, the current leakage is taken into account in the
From above literatures, it can be observed that there are electrochemical model to provide accurately performance of
wider ranges of fuel options; both non-renewable and SOFCeHþ.
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Fig. 2 e Schematic of a steam reformer and SOFCeHþ integrated system.

Description of external reformer and SOFCeHþ

integrated system Table 1 e Specification data of each unit used in an
integrated system of a steam reformer and SOFCeHþ.
Fig. 2 presents the integrated system of external reformer and
Name Unit Model Standard Operating Condition
SOFCeHþ designed in Aspen Plus simulator. The reforming
process considered in this work is steam reforming because it Steam reformer
MIXER Mixer e
can provide the highest hydrogen amount compared with
HEATER Heater 473 K
other methods [28]. From Fig. 2, FUEL stream (i.e., ethanol, REFORMER RGibbs 1073 K, 1 bar
glycerol and biogas) and STEAM stream are firstly mixed SOFCeHþ
through mixer (MIXER) before sending to the heater (HEATER) ANODE RGibbs 973 K, 1 bar
to increase temperature at a given value. Then, EX-HEAT CATHODE RGibbs 973 K, 1 bar
stream as pre-heated stream is fed into the reformer
(REFORMER) where the steam reforming reaction is accom-
CH4 4 C þ 2H2 (13)
plished depending on the types of fuel. The possible reactions
occurred in the steam reformer when ethanol, glycerol and It is noted that the water gas-shift reaction (Eq. (6)) can be
biogas are used as feed are shown below: occurred in steam reformer when biogas and glycerol are used as
Ethanol reactant. Then, the gas product (REFORMED stream) obtained
from reformer is further sent to the anode side of SOFCeHþ
C2H5OH þ 2H2O 4 2CO2 þ 6H2 (3) (ANODE) to generate the electricity. Due to the residual of CO,
CH4, and steam, the water gas shift (Eq. (6)) and the methane
C2H5OH þ H2O 4 CO2 þ 4H2 (4) steam reforming (Eq. (12)) reactions may be promoted on the
anode side. This can increase hydrogen concentration for elec-
C2H5OH 4 CH4 þ CO þ H2 (5) trochemical reaction. Then, hydrogen at the anode side is
oxidized where protons and electrons are produced. The protons
CO þ H2O 4 CO2 þ H2 (6) migrate through the electrolyte to the cathode side (CATHODE)
where they react with oxygen in AIR stream and here steam can
CO þ 3H2 4 CH4 þ H2O (7) be generated. The flow of electron through the external circuit
can provide the direct-current electricity. Since Aspen Plus
CO2 þ 4H2 4 CH4 þ 2H2O (8) simulator cannot model the transfer of ions and thus, the overall
electrochemical reaction of SOFCeHþ (Eq. (14)) is used to replace
Glycerol the cell half reactions [29]. In case of SOFCeHþ, this reaction takes
place in the cathode side (CATHODE) where steam is generated.
C3H8O3 þ 3H2O 4 3CO2 þ 7H2 (9)
H2 þ 1/2O2 / H2O (14)
C3H8O3 4 3CO þ 4H2 (10)

Biogas
Methodology
CH4 þ CO2 4 2CO þ 2H2 (11)
In this work, the thermodynamic analysis is conducted
CH4 þ H2O 4 CO þ 3H2 (12) through Aspen Plus simulator to analyse the performance of
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an integrated system with using different fuel types. The main current density caused from current leakage (ileak) as
assumptions used to develop are included: the system is following equation:
operated under isothermal and steady state conditions; it has
i ¼ iexternal þ ileak (17)
not pressure drop during unit operation; all chemical re-
actions are reached to chemical equilibrium, and only H2 is Based on the data of Zhang et al. [31], it was found that the
oxidized at the anode side of SOFCeHþ. Table 1 lists specifi- current leakage is a function of temperature as shown in Eq.
cation data of each unit used in an integrated system. When (18)
the operating conditions of reformer (i.e., temperature and  
pressure) are specified with a fixed molar ratio of steam and 7; 607
ileak ¼ 0:0614 exp (18)
carbon (S/C ratio), the content of reformate can be firstly T
determined. Here, steam reformer is represented by RGibbs Next, the equations used for the computation of each
reactor model in which the equilibrium composition of voltage loss are shown below:
reformate can be computed by using the minimization prob-
lem of the Gibbs free energy. The thermodynamic method - Activation overpotential
used in the calculation is Soave-Redlich-Kwong (SRK) [30].
Next, reformate is delivered to the anode side of SOFCeHþ
whereas air is fed to the cathode side. Since both electrodes  
take place the chemical reaction, they can be modelled by 2<T 1 i
hact;an ¼ sinh (19)
RGibbs reactor model. Besides the specification of SOFCeHþ nF 2i0;an
operating condition, according to Table 1, the electrochemical  
equations are added through a calculator block in Aspen Plus 2<T 1 i
hact;ca ¼ sinh (20)
simulator to calculate the performance of SOFCeHþ. Cell nF 2i0;ca
dimension, material properties and other operating condi- where hact;an and hact;ca are the activation overpotential at the
tions of SOFCeHþ, as listed in Table 2 are also input parame- anode and cathode sides, respectively. n is the number of
ters for the simulation. electrons transferred in the single elementary rate-limiting
In this work, the anode, electrolyte and cathode of reaction and i0,an and i0,ca are the exchange current density
SOFCeHþ are made from NiO-BZCY, BZCY and LNO-LNF, of each electrode. Since it has not been reported on the ex-
respectively. As a given the operating condition of SOFCeHþ, change current density of NiO-BZCY (anode) and LNO-LNF
i.e., temperature, pressure and current density corresponding (cathode), they are determined in the model validation, as
to other parameters, the electrical performance of SOFCeHþ described in Section Model validation.
can be determined which will be shortly described as follow:
The compositions of H2 in IN-CAT stream and O2 in AIR - Ohmic loss
stream are used to compute an open-circuit voltage (E), as
expressed in Eq. (15).
! tele
hohm ¼ i (21)
<T pH2 OðcaÞ sele
E ¼ E0  ln (15)
2F pH2 ðanÞ p0:5
O2 ðcaÞ where tele indicates the electrolyte thickness and sele repre-
Since the steam is generated at the cathode side of sents the summation of protonic and electronic conductivity
SOFCeHþ. In order to avoid the misunderstanding, the sub-
scription of “an” and “ca” are used to represent the partial Table 2 e Input parameters of SOFCeHþ used in the
pressure of specie i at anode and cathode, respectively. E0 is simulation.
the open-circuit voltage as a function of operating tempera- Parameters Value
ture at the standard pressure.
Operating conditions
However, when the current is drawn from the SOFCeHþ,
Current density (i) 1 A/cm2
the cell voltage (V) will be dropped from the open-circuit Anode diffusion coefficient 1.05  104 m2/s
voltage. This is due to three main losses occurred in the ðDan;eff Þ
SOFCeHþ which are composed of (1) the activation over- Cathode diffusion coefficient 4.49  105 m2/s
potential (hact) caused by electrochemical reactions at the ðDca;eff Þ
electrode, (2) the ohmic loss (hohm) mainly depended on the Dimensions of cell element
Active area of cell (Acell) 40 m2
ionic conductivity of electrolyte and (3) the concentration
Anode thickness (tan) 500 mm
overpotential (hconc) occurred by mass transfer limitations on Cathode thickness (tca) 50 mm
the reactants at the interface of electrode and electrolyte. Electrolyte thickness (tele) 20 mm
Thus, the actual cell voltage (V) is always lower than the open- Material properties  
4:47  103 1; 892
circuit voltage, as Eq. (16). Electrolyte protonic exp U1 ,m1
T T
conductivity (sp)  
5; 815
V ¼ E  ðhact þ hohm þ hconc Þ (16) Electrolyte electronic 1:449  103 exp U1 ,m1
T
conductivity (se)
For the SOFCeHþ with mixed-conducting electrolyte, the Electrode porosity (ε) 0.4
total current density (i) is the summation of the current den- Electrode pore radius (r) 0.5 mm
sity performed through the external circuit (iexternal) and the Electrode tortuosity (x) 5.0
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of BZCY electrolyte. In this study, the protonic conductivity of validation is separately performed. In part of steam reformer
BZCY was extracted from Ding et al. [32] whereas its electronic with different fuel types, there are many researchers pro-
conductivity was estimated from Heras-Juaristi et al. [26]. posed the comparison results of gas product obtained from
These values are shown in Table 2. the simulation and experimental data. It is found that the
simulation results obtained from Aspen Plus simulator agree
- Concentration overpotential with the experimental data when ethanol [34], glycerol [35]
and biogas [36] are used as reactant in the steam reformer. For
! the SOFCeHþ, the proposed model in Section Methodology is
<T pH ðanÞ validated with experimental data of Hou et al. [37]. In this
hconc;an ¼ ln I 2 (22)
2F pH2 ðanÞ experiment, the materials used in SOFCeHþ for anode, elec-
trolyte and cathode are from NiO-BZCY|BZCY|LNO-LNF with
!0:5 ! their thicknesses as 1000 mm, 20 mm, and 20 mm, respectively.
<T pO2 ðcaÞ pIH2 OðcaÞ
hconc;ca ¼ ln (23) Fuel is composed of 97% H2 and 3% H2O whereas oxidant
2F pIO2 ðcaÞ pH2 OðcaÞ
consist of 21% O2 and 79% N2. As mentioned above, there is no
where hconc;an and hconc;ca are the concentration overpotential open literature expressed the exchange current density of
at the anode and cathode sides, respectively. pIH2 , pIO2 and pIH2 O NiO-BZCY (anode) and LNO-LNF (cathode) and thus, they are
estimated from model validation. Fig. 3 shows the comparison
are the partial pressures of hydrogen, oxygen and steam at the
of cell voltage and power density obtained from the model
interface which can be derived from gas transport model in
prediction and experimental data as SOFCeHþ operated at
the porous electrodes as follows:
temperature of 973 K and pressure of 1 atm. It can be seen that
   
i<Ttan the model prediction gives a good agreement with the
pIH2 ¼ Pan  Pan  pH2 ðanÞ exp (24)
2FDan;eff Pan experimental data when the exchange current densities of
anode and cathode are 1.2 and 0.9 A/cm2, respectively.
i<Ttca
pIO2 ¼ pO2 ðcaÞ  (25)
2FDca;eff
Results and discussion
i<Ttca
pIH2 O ¼ pH2 OðcaÞ þ (26)
4FDca;eff This work aims to analyse the performance of power gener-
ation consisting of steam reformer and SOFCeHþ. Although
where tan and tca are the thickness of anode and cathode,
each unit can be separately operated, the operation of
respectively, and Dan,eff and Dca,eff represent the effective gas
reformer strongly affects to the SOFCeHþ. Thus, the operation
diffusivity coefficients in the anode and cathode sides. It is
of each unit must be optimized to provide the best integrated
noted that the detailed electrochemical equations of
system performance. This section can be divided into two
SOFCeHþ were explained in our previous work [33].
sections. The first section (Section Hydrogen production from
When the actual cell voltage of SOFCeHþ is calculated from
steam reformer) presents the performance analysis of steam
Eq. (16), the performance of SOFCeHþ in terms of power
reformer with respect to a wider range of operating conditions
density (Pw), SOFCeHþ electrical efficiency ðεSOFCHþ Þ, system
and different fuel types. Then, reformate obtained from the
electrical efficiency ðεsys Þ, and fuel utilization (Ufuel), can be
suitable operating condition is fed into the anode side while
determined as below equations.
air is entered to the cathode side and thus, the electrical
Pw ¼ iV (27) performance can be determined. In Section SOFC and system

iVAcell
εSOFCHþ ¼  100% (28)
n_CH4;in LHVCH4 þ n_H2;in LHVH2 þ n_COin LHVCO

iVAcell
εsys ¼    100% (29)
n_Fuelin LHVFuel

iAcell
Ufuel ¼   (30)
2F 4n_CH4;in þ n_COin þ n_H2;in

where n_i;in represents the molar flow rate of specie i fed to the
SOFCeHþ whereas n_Fuelin is the molar flow rate of fuel (i.e.,
ethanol, glycerol or biogas) supplied to the integrated system.

Model validation
Fig. 3 e Comparison of cell voltage and power density of
Since the investigation of a steam reformer and SOFCeHþ
SOFCeHþ obtained from model prediction and
integrated system has not been published before, the model
experimental data [37].
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performances, the effect of operating conditions of SOFCeHþ value at temperature above 1073 K since the chemical equi-
fuelled by different reformates on the cell and system per- librium is limited at this temperature. From the simulation
formances is explained. result, although the highest H2 mole fraction can be obtained
at 1073 K, the H2 mole fraction hardly differs from steam
Hydrogen production from steam reformer reformer operated at 973 K. In order to eliminate the energy
demand, the operation of steam reformer at 973 K is more
In this section, the influence of reformer temperature on suitable.
hydrogen production is investigated. The reformer tempera- Besides H2 production, the carbon formation in the steam
ture studied is adjusted in range of 773e1173 K while other reformer should be considered to prevent the carbon deposi-
parameters in Table 1 are kept constant. In this study, the S/C tion on the catalyst, leading to the catalyst deactivation.
ratio used in this simulation equals to 1. Thus, steam fed into Fig. 4b shows the carbon mole fraction as a function of
the reformer is fixed at 1 kmol/h whereas molar flow rates of reformer temperature. The simulation result shows that
ethanol, glycerol and biogas are set as 0.5, 0.33 and 1 kmol/h, higher temperature operations can supress carbon formation.
respectively. Fig. 4a demonstrates H2 mole fraction as a Since the Boudouard reaction (Eq. (1)) and reverse coke gasi-
function of reformer temperature at different fuel options. fication reaction (Eq. (2)) are exothermic reactions which are
The simulation results reveal that the trend of H2 mole frac- favorable to operate at low temperature. Thus, when the
tion obtained from each fuel type is similar. Hydrogen pro- reformer is operated at higher operating temperature, both
duction can be improved with an increase in reformer reactions can shift toward the reactant side and this results in
temperature because the steam reforming reaction as endo- a decrease in carbon formation. From the result, it can be
thermic reaction is more pronounced at higher operating observed that in temperature range of 773e973 K, the order of
temperature. However, H2 mole fraction reaches constant carbon formation is glycerol < biogas < ethanol. Glycerol
shows the less carbon formation since glycerol contains a
high ratio of O2 and carbon, the conversion to CO and CO2 can
avoid the carbon formation. Unlike glycerol, biogas is mainly
composed of CH4 whereas ethanol has the tendency of CH4
production. The presence of CH4 in the reformer can cause the
additional carbon formation through Eq. (13). In addition, it is
found that there is no carbon formed at the selected reformer
temperature as 973 K for all fuel types since the carbon for-
mation thermodynamically hindered at temperature 973 K
and above. This result is coherent with Montero et al. [38].
Under the reformer operation as 973 K and 1 bar with S/C
ratio of 1, each fuel type can provide reformate with different
gas composition as shown in Fig. 5. Although CH4 and CO are
residues for all fuel types, these gaseous can be further con-
verted into H2 via steam reforming and water gas shift re-
actions on the anode side of SOFCeHþ. Under this operation as
seen in Fig. 5, the use of ethanol in steam reformer can provide
58 mol% H2 as the highest H2 production, followed by biogas

Fig. 4 e Mole fractions of: (a) H2 and (b) carbon as a function Fig. 5 e Reformate composition obtained from using
of reformer temperature at different fuel options (S/C different fuels at temperature of 973 K and S/C ratio of 1.
ratio ¼ 1).
11452 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 1 1 4 4 5 e1 1 4 5 7

and glycerol as 52 mol% and 50 mol%, respectively. Consid- from 973 to 1073 K. Thus, the suitable SOFCeHþ temperature
ering the energy consumption of the reformer as calculated should be selected at 973 K to avoid the fabrication and
from Eq. (31), the result indicates that more H2 production operation cost as well as cell durability.
obtained from more pronounced reforming reactions leads to From the mole fraction of reformate as shown in Fig. 5, the
higher total heat duty of reforming section. Thus, the ethanol order of H2 contained in reformate is
reformer requires the largest external heat as 67 kW whereas ethanol > biogas > glycerol. In general, the H2 mole fraction
the biogas and glycerol reformers use energy as 65 and 57 kW, affects to the open-circuit voltage and concentration loss.
respectively. Higher H2 mole fraction tends to improve the open-circuit
! ! voltage whereas the concentration loss is reduced. As ex-
X outlet outlet X in in
QSR ¼ n_i;R h_i;R  n_i;R h_i;R (31) pected, the cell voltage and power density should be
R R enhanced. However, there is a few differences of H2 mole
fraction in each reformate. At constant temperature operation
where n_i;R and h_i;R represent the molar flow rate and the
and current density, the variation of cell voltage and power
enthalpy of species i that relate to the reformer.
density cannot be observed when the different reformates are
used. This is a reason why Fig. 6a shows only one curve as the
SOFC and system performances
cell voltage and power density obtained from using ethanol
reformate.
Before the cell performance and overall system efficiency are
Unlike cell voltage and power density, the improved
determined, the possibility of carbon formation on the anode
SOFCeHþ and system electrical efficiencies have variation
side of SOFCeHþ should be firstly considered. This is because
when the SOFCeHþ is fuelled by different reformate types, as
there are some amounts of CH4 and CO fed into the anode
seen in Fig. 6b. The result indicates that the SOFCeHþ fuelled
side; not only steam reforming and water gas shift reaction
by glycerol reformate can provide the highest cell efficiency,
can be occurred but also carbon formation via Eqs. (1), (2) and
followed by using biogas and ethanol reformates. This can be
(13) may be accomplished. From the simulation result, it is
found that there is no carbon formed on the anode side of
SOFCeHþ for all reformates obtained at 973 K with S/C ratio of
1.
When the appropriate operating condition of reformer
from Section Hydrogen production from steam reformer is
specified corresponding with the SOFCeHþ operating condi-
tion shown in Table 2, the cell performance and overall sys-
tem efficiency can be further determined. It is noted that the
inlet molar flow rate of air consisting of 21% O2 and 79% N2 is
1 kmol/h. Fig. 6 illustrates the impact of SOFCeHþ tempera-
ture (773, 873, 973 and 1073 K) on cell voltage, power density,
cell efficiency, and system efficiency with different reformate
types. The simulation results indicate that increasing
SOFCeHþ temperature can improve both cell and system
performances for all types of reformate. Higher temperature
operation can improve an open-circuit voltage whereas ohmic
loss is reduced. Since the exchange current densities and
effective diffusivities of electrodes are constant values which
are independent of temperature, the activation and concen-
tration overpotentials becomes higher as higher operating
temperature. However, these values are very small compared
to the ohmic loss. It is noted that the exchange current den-
sities of electrodes are obtained from model validation at
SOFCeHþ temperature of 973 K and thus, the results of per-
formance in the outer range may be deviated. Due to higher
open-circuit voltage and lower ohmic loss, the cell voltage and
power density can be enhanced as seen in Fig. 6a. The
improvement of power density leads to a significant increase
in cell efficiency (solid line) and system efficiency (dash line)
as illustrated in Fig. 6b. It is noted that Eff_ethanol, Eff_gly-
cerol and Eff_biogas represent the cell efficiency obtained
from using ethanol, glycerol and biogas reformates whereas
Fig. 6 e Effect of SOFCeHþ temperature on: (a) cell voltage
Sys_ethanol, Sys_glycerol and Sys_biogas are the value of
and power density obtained from using ethanol reformate
system efficiency from fuelled by ethanol, glycerol and biogas.
and (b) cell and system efficiencies for all types of
However, it can be observed from Fig. 6a that the cell voltage
reformate at current density of 1 A/cm2.
will be dropped when the SOFCeHþ temperature is raised
 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 1 1 4 4 5 e1 1 4 5 7 11453

explained by the difference in fuel amount fed into the Next, the influence of current density on cell and system
SOFCeHþ stack which is referred to molar flow rate of CH4- performances is investigated when the SOFC is operated at
þ H2 þ CO. The simulation result at SOFC temperature of 973 K constant temperature of 973 K. As seen in Fig. 8a, increasing
and current density of ~1 A/cm2 reveals that the order of fuel current density, in a range of 0.6e1.2 A/cm2, leads to a
amount fed into the SOFCeHþ stack is decrease in cell voltage due to the increment of all voltage
ethanol > biogas > glycerol. Compared to others, glycerol losses that include activation overpotentials, ohmic loss, and
reformate contains less amount of fuel whereas the electricity concentration overpotentials. Unlike cell voltage, the power
production (referred to current density) is always constant.
Consequently, the conversion of chemical energy into elec-
trical energy or electrical efficiency provided from glycerol
reformate is the highest. In addition, it is found that the
highest system efficiency can be obtained when the glycerol
reformate is used; this is since the inlet molar flow rate of
glycerol fed in the integrated system is the lowest compared to
the others. Interestingly, the order of system efficiency ob-
tained from different reformates is
glycerol > ethanol > biogas. This is because biogas must be fed
into system as highest flow rate to keep the S/C ratio of 1 and
thus, the integrated system fuelled by biogas has the lowest
system efficiency.
Considering the different types of reformate as Fig. 7, the
use of glycerol reformate can provide the highest fuel utili-
zation (90%), followed by biogas (86%) and ethanol (69%). In
addition, Fig. 7 also shows the exhaust gas distribution ob-
tained from using different reformates. It can be observed that
the CO content in exhaust gas is quite high for all reformate
types. This is mainly caused by the generation of steam from
electrochemical reaction at the cathode. There is no steam to
promote the water gas shift reaction and thus, high amount of
CO is presented. In general, the residual CO is recommended
to combust with excess O2 from the cathode in an afterburner.
High temperature gas obtained from afterburner can be sup-
plied to other unit operations in the SOFCeHþ system, and this
leads to an improved system thermal efficiency. Besides the
CO combustion, valuable gas content may be further used in
other units and this can improve the overall system efficiency.
Furthermore, it is observed that the integrated system with
ethanol reformate has more amount of residual fuel
compared to the others.

Fig. 8 e Effect of current density on: (a) cell voltage and

Fig. 7 e Fuel utilization and exhaust gas composition power density obtained from using ethanol reformate, (b)
obtained from using different reformates (SOFCeHþ cell and system efficiencies, and (c) fuel utilization for all
temperature ¼ 973 K and current density ¼ 1 A/cm2). types of reformate at SOFCeHþ temperature of 973 K.
11454 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 1 1 4 4 5 e1 1 4 5 7

density can be enhanced when the SOFC is operated at higher electricity; this means more fuel consumed to provide elec-
current density. The raised power density can further improve tricity. Thus, the tendency of fuel utilization with different
the cell and system efficiencies, as demonstrated in Fig. 8b. In reformates is glycerol > biogas > ethanol. However, it can be
case of fuel utilization (Fig. 8c), the fuel utilization is higher observed that the current density used for glycerol and biogas
with increasing current density. This indicates that the fuel is reformates is limited at 1.1 A/cm2. At the current density
more consumed to produce electricity when current density is above 1.1 A/cm2, reformate is rapidly consumed to generate a
higher. constant electricity; this is mainly affected on concentration
The trend of results with using different reformates is like overpotentials and thus, the cell performance of SOFCeHþ
the temperature effect. At constant current density, there are may be a rapid drop. In order to prevent this situation, the
no differences in cell voltage and power density when selected current density should not be more than this value.
different reformates are used. Thus, Fig. 8a shows the cell Due to the discrepancy of cell and system performances,
voltage and power density obtained from using ethanol the selection of favoured operating condition must be opti-
reformate. Considering the cell and system efficiencies mized these values. Besides electrical performances, the
(Fig. 8b), it can be observed that the use of glycerol reformate exhaust gas composition and energy used or provided are
can provide the largest amount of cell and system efficiencies other factors that should be considered. Figs. 7 and 9 show the
compared to the others. However, the cell efficiency obtained content of H2, CO, CO2 and H2O at the exit of SOFCeHþ when
from using biogas reformate is higher than that from ethanol the SOFCeHþ is operated at different reformates and current
reformate whereas the system efficiency of ethanol reformate densities. The results show that although the SOFCeHþ
is superior to that of biogas reformate. This is mainly due to operation with high current density can consume more H2 to
the differences in molar flow rate of fuel fed into SOFCeHþ produce electricity which is referred to higher efficiency and
and integrated system. When the fuel utilization of each fuel utilization, this operation provides the exhaust gas con-
reformate is considered, it is found from Fig. 8c that reformate taining higher amount of CO. In addition, it can be observed
from glycerol has the highest fuel utilization, followed by that the use of ethanol reformate can provide the exhaust gas
biogas and ethanol. The main reason is the molar flow rate of
CH4, CO and H2 fed into the SOFCeHþ from glycerol reformate
is lower than the others. The SOFCeHþ is fed with lower fuel
flow rate while the SOFCeHþ can produce constantly

Fig. 10 e Energy consumption of (a) SOFCeHþ stack and (b)

Fig. 9 e Exhaust gas composition obtained from using integrated system obtained from using different
different reformates and current densities: (a) 0.9 A/cm2 reformates and current densities (SOFCeHþ
and (b) 1.1 A/cm2 (SOFCeHþ temperature ¼ 973 K). temperature ¼ 973 K).
 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 6 ( 2 0 2 1 ) 1 1 4 4 5 e1 1 4 5 7 11455

consisting of highest H2 and CO contents due to its lowest fuel fuel utilization. Then, the effect of current density on exhaust
utilization. gas composition and energy consumption obtained from in-
Fig. 10 presents the energy consumptions of SOFCeHþ and tegrated system was examined. The simulation results
integrated system when SOFCeHþ is operated at different showed that the higher current density operation can provide
current densities. The results demonstrated in Fig. 10a reveal higher amount of CO in the exhaust gas. Thus, the current
that the SOFCeHþ can provide more heat when it is operated density as moderate value of 1 A/cm2 is selected to optimize
at higher temperature. This is since the electrochemical re- both performances and gas composition exited from system.
action as exothermic reaction can be strongly carried out with Under the suitable operation of SOFCeHþ (temperature of
increasing temperature. When the energy consumption of 973 K and current density of 1 A/cm2), it was found that the
integrated system is studied as Fig. 10b, it is found that the system efficiency of using glycerol reformate was superior to
integrated system can provide heat less than the SOFCeHþ the others. In addition, the integrated system fuelled by
stack due to the addition of heat duty in reforming section. glycerol can provide the largest heat to the surrounding.
When the difference in reformate types is considered, it is Therefore, it is clearly shown that glycerol is suitable renew-
found that the SOFCeHþ fuelled by glycerol reformate releases able fuel for SOFCeHþ integrated system.
heat to the surrounding more than the others. This can be
explained from high conversion of fuel to electricity.
From above simulation result, although the operation at
Declaration of competing interest
high current density ~1.1 A/cm2 can obtain high power den-
The authors declare that they have no known competing
sity, efficiency, fuel utilization and heat production for all
financial interests or personal relationships that could have
reformate types, this operation may cause the high CO con-
appeared to influence the work reported in this paper.
tent released to the atmosphere. Therefore, the selection of
current density around 1 A/cm2 is recommended in this work.
The simulation results also reveal that the integrated system
using glycerol provides the best performance in terms of ef- Acknowledgements
ficiency and fuel utilization. Moreover, this system releases
high amount of heat to the surrounding while the CO is The supports from King Mongkut’s Institute of Technology
emitted less than the others. Ladkrabang and the National Research Counsil of Thailand are
gratefully acknowledged.

Conclusions
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Greek letters hohm: Ohmic overpotential (V)

se: Electronic conductivity of electrolyte (1/U m)
εSOFCHþ : SOFCeHþ electrical efficiency sele: Total conductivity of electrolyte (1/U m)
εsys : System electrical efficiency sp: Protonic conductivity of electrolyte (1/U m)
hact,an: Activation overpotential at the anode (V) tan: Anode thickness (m)
hact,ca: Activation overpotential at the cathode (V) tcat: Cathode thickness (m)
hconc,an: Concentration overpotential at the anode (V) telec: Electrolyte thickness (m)
hconc,ca: Concentration overpotential at the cathode (V)