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 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 43, 2015 

A publication of 

The Italian Association 
of Chemical Engineering 
Online at www.aidic.it/cet 

Chief Editors: Sauro Pierucci, Jiří J. Klemeš 
Copyright © 2015, AIDIC Servizi S.r.l., 
ISBN 978-88-95608-34-1; ISSN 2283-9216                                                                               

 

Design of the Integrated Solid Oxide Fuel Cell and Molten 
Carbonate Fuel Cell System to Reduce Carbon Dioxide 

Emissions 

Prathak Jienkulsawada, Dang Saebeab, Yaneeporn Patcharavorachotc, Amornchai 
Arpornwichanop*a 
aComputational Process Control Research Unit, Department of Chemical Engineering, Faculty of Engineering, 
Chulalongkorn University, Bangkok 10330, Thailand  
bDepartment of Chemical Engineering, Faculty of Engineering, Burapha University, Chonburi 20131, Thailand  
cSchool of Chemical Engineering, Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok 
10520, Thailand 
Amornchai.a@chula.ac.th 

In general, a solid oxide fuel cell (SOFC) based on an internal reforming operation cannot be run with 
complete fuel utilization; therefore, the remaining fuel needs to be effectively handled. Furthermore, the SOFC 
exhaust gas still contains carbon dioxide, which is the primary greenhouse gas, and searching for the way to 
utilize this carbon dioxide is important. A molten carbonate fuel cell (MCFC) appears to be a potential 
technology to mitigate the emissions of carbon dioxide. In this study, the performance of the integrated SOFC 
and MCFC system is analyzed. The SOFC is considered a main power generation and the MCFC is regarded 
as a carbon dioxide concentrator along with producing electricity as a by-product. Mathematical models of the 
SOFC and MCFC are based on one-dimensional mass balances taking into all various cell voltage losses 
under steady-state and isothermal conditions. Primary operating conditions of the integrated fuel cell system 
that affects the system efficiencies in terms of power generation and reduction in the carbon dioxide emission 
are discussed and its optimal operation is identified based on these criteria. Effect of carbon dioxide 
recirculation on the system is also studied. Various configurations of the integrated SOFC-MCFC system are 
proposed and compared to determine the suitable design of the integrated fuel cell system. 

1. Introduction 

Fuel Cells are regarded as alternative power generation device that can directly convert chemical energy in 
fuel into electrical energy with high efficiency and environmental friendliness, compared to a conventional 
combustion-based process (Chatrattanawet et al., 2014). In general, types of fuel cells are classified by their 
operating temperatures. Solid oxide fuel cell (SOFC) and molten carbonate fuel cell (MCFC) are the high-
temperature fuel cells (HTFC) that are typically operated at temperature over 600 °C. At this operating 
condition, heat produced from the fuel cells can be used in fuel processing and heat generation systems 
(McPhail et al., 2011).  
As a hydrogen deficiency in cell stack causes collapse in physical structure, SOFC is basically operated at 
moderate fuel utilization (Saebea et al., 2014). Under this operation, the anode exhaust gas is still valuable 
because of remaining fuels, such as hydrogen and carbon monoxide. These useful fuels can be directly used 
in MCFC for producing additional power (Pastorino et al., 2011). Moreover, MCFC needs carbon dioxide 
circulation in its system; this means the MCFC appears to be a potential technology to mitigate the emissions 
of carbon dioxide (Wee, 2014). Hence, the integrated system of SOFC and MCFC is not only potential solution 
for increasing fuel utilization and power, but also reduces the carbon dioxide emissions. Different configuration 
designs of the SOFC-MCFC integrated system are possible. In this study, performance of such the fuel cell 

                                

 
 

 

 
   

                                                  
DOI: 10.3303/CET1543366 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Jienkulsawad P., Saebea D., Patcharavorachot Y., Arpornwichanop A., 2015, Design of the integrated solid oxide 
fuel cell and molten carbonate fuel cell system to reduce carbon dioxide emissions, Chemical Engineering Transactions, 43, 2191-2196   
DOI: 10.3303/CET1543366

                                

 
 

 

 
   

                                                  
DOI: 10.3303/CET1543366 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Jienkulsawad P., Saebea D., Patcharavorachot Y., Arpornwichanop A., 2015, Design of the integrated solid oxide 
fuel cell and molten carbonate fuel cell system to reduce carbon dioxide emissions, Chemical Engineering Transactions, 43, 2191-2196   
DOI: 10.3303/CET1543366

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integrated system with different designs is investigated and compared in terms of power generation and 
carbon dioxide emissions.  

2. System configuration 

A planar SOFC consists of Ni-YSZ as anode, YSZ as electrolyte and YSZ-LSM as cathode. For MCFC, Ni-
alloy is used as anode, Li2CO3/Na2CO3 is used as electrolyte and NiO is used as cathode. In the fuel cell 
integrated system, SOFC acts as a main power generation and MCFC as a carbon dioxide concentrator along 
with producing electricity as a by-product. Reformate gas from the reforming process is fed into the SOFC in 
order to maximize the power generation. The remaining fuels from the SOFC are introduced to the MCFC. In 
this study, three different configurations of the integrated system, as shown in Figure 1, are analyzed and 
compared to determine the suitable system design in terms of power generation and carbon dioxide emission. 
For the first configuration (A)(Figure 1(a)), the product of the SOFC is directly fed to the MCFC and portion of 
the exhaust gas from an after-burner is recirculated to the MCFC. This configuration is aimed at operating 
MCFC under less fuel concentration because of less fuel content in product of SOFC. In the second design 
(B)(Figure 1(b)), the reformate gas is separately fed to SOFC and MCFC. The anode off-gas from SOFC and 
MCFC are mixed with remaining gas from the cathode of MCFC and burnt in the after-burner and portion of 
the exhausted gas is recirculated. Configuration (B) aims to study the effect of increasing in feed to MCFC and 
consequently reduce feed to SOFC for increasing the amount of fuel feed to operate MCFC. In the third 
system configuration (C)(Figure 1(c)), the reformed gas is separately fed to SOFC and MCFC. The remaining 
gas from SOFC are mixed and burnt in the after-burner. The product gas from after-burner is fed to the 
cathode of MCFC. The anode off-gas from MCFC is fully recirculated. In the third configuration, the aim is 
adding MCFC on the downstream of the existing SOFC system by dividing the feed of SOFC to operate 
MCFC. 

3. Mathematical model 

The reactions that take place in the reformer, SOFC and MCFC are shown in Table 1. In the reformer, only the 
steam reforming reaction and water-gas shift reaction are considered and assumed to be at equilibrium. In 
SOFC and MCFC, the electrochemical reactions occur to generate electricity. A complete combustion reaction 
is assumed at the after-burner. Modelling of the SOFC and MCFC is based on one-dimensional mass balance 
and electrochemical model under steady-state and isothermal conditions. 

3.1 Mass balance 

Mass balance of gaseous components at the anode and cathode of the SOFC and MCFC can be written as 
below: 

 

Figure 1: Configurations of the SOFC and MCFC integrated system 

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Table 1:  List of reactions 
Steam reforming (i) 4 2 2CH H O 3H CO+ ↔ +  
Water-gas shift (ii) 2 2 2CO H O H CO+ ↔ +  
Hydrogen oxidation (iii) 2

2 2H O H O 2e
− −+ → +  

Oxygen reduction (iv) 2
20.5O 2 Oe

− −+ →  
Overall cell (SOFC) (v) 

2, 2, 2 ,H 0.5O H Oa c a+ →  
Hydrogen oxidation (vi) 2

2 3 2 2H CO H O CO 2e
− −+ → + +  

Oxygen reduction (vii) 2
2 2 30.5O CO 2 COe

− −+ + →  
Overall cell (MCFC) (viii) 

2, 2, 2, 2, 2 ,H 0.5O CO CO H Oa c c a a+ + → +  

 

, k
i

i k
k

F
W R

x
ν

∂  
=  ∂  

   (1) 

where F is the molar flow rate (mol s-1), W is the cell width (m), ν is the stoichiometric coefficient of 
component i in the reaction k and Rk is the rate of reaction (mol m-2 s-1). 
The rates of reactions (Rk) in the SOFC are expressed as Eqs(2)-(4) for the steam reforming, water-gas-shift 
and electrochemical reaction, respectively (Patcharavorachot et al., 2010). 

4 2

1.25
0 CH H O exp

a
SR

E
R k p p

T
−  = − ℜ 

  (2) 

2 2

2

CO H
CO H OWGS WGSR a

eq

p p
R k p p

K
τ
 

= −  
 

 (3) 

2E SOFC

j
R

F
 =  
 

 (4) 

where WGSRk is the pre-exponential factor (mol m-3 Pa-2 s-1) and eqK is the equilibrium constant given by: 

103191
0.0171expWGSRk T

 = − ℜ 
 (5) 

( )3 2exp 0.2935 0.6351 4.1788 0.3169
1000

1

eqK Z Z Z

Z
T

= − + + +

= −
 (6) 

For the MCFC, the rates of reactions can be shown as Eqs(7)-(9) for the steam reforming, water-gas-shift and 
electrochemical reaction, respectively.  

2

4 2

3
CO H ,2

CH H O
,1

1 298.15
exp SRSR SR

SR SR

y y a
R Arr y y

a T K LW

    
= − × − ×            

  (7) 

2 2

2

CO H , 2
CO H O

,1

1 298.15
exp WGSWGS WGS

WGS WGS

y y a
R Arr y y

a T K LW

    
= − × − ×          

 (8) 

2E MCFC

j
R

F
 =  
 

 (9) 

Ovrum and Dimopoulos (2012) proposed the respective equilibrium coefficients ( SRK and WGSK ) and the values 

of the numerical constants ( ,1 , 2 WGS,1 WGS, 2, , , , ,SR WGS SR SRArr Arr a a a a ). 

  

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3.2 SOFC electrochemical model 

The open-circuit voltage ( OCVE ) of the SOFC is determined by Nernst equation: 

2

2 2

H O0
0.5

H O

ln
2OCV

pRT
E E

F p p
 

= −   
 

 (10) 

where 0E is the open-circuit voltage at standard pressure and it depends on temperature as shown in Eq(11) 
0 41.253 2.4516E e T−= −  (11) 

However, there are internal losses inside the SOFC stack that make the actual voltage lower than the open-
circuit called operating voltage (E) as (Aguiar et al., 2004): 

OCV lossE E η= −   (12) 

The concentration overpotentials ( concη ) can be expressed as: 

2 2

2 2

H O, H ,
conc,anode

H O, H ,

ln
2

TPB f

f TPB

p pRT
F p p

η
 

=   
 

 (13) 

2

2

O ,
,cathode

O ,

ln
4

a
conc

TPB

pRT
F p

η
 

=   
 

 (14) 

The partial pressures (atm) at the three-phase boundaries (TPB) determined by using a gas transport model in 
porous material are shown in Eqs(15)-(17). 

2 2H , H ,
,2

a
TPB f

eff a

RT
p p j

FD
τ

= −   (15) 

2 2H O, H O,
,2

a
TPB f

eff a

RT
p p j

FD
τ

= +  (16) 

( )
2 2O , O ,

,

exp
4

c
TPB a

eff c

RT
p P P p j

FD P
τ 

= − −   
 

 (17) 

The activation overpotentials ( actη ) can be determined by the non-linear Butler-Volmer equation as follows: 

2 2

2 2

H , H O,
0, ,anode ,anode

H , H O,

(1 )
exp expTPB TPBanode act act

f f

p pnF nF
j j

p RT p RT
α α

η η
 −   = − −    

     
 (18) 

0,cathode ,cathode ,cathode
(1 )

exp expact act
nF nF

j j
RT RT

α α
η η

 −   = − −    
    

 (19) 

where 0,cathodej and 0,anodej are the exchange current density (A m-2) at the cathode and anode expressed as: 

{ }

electrode
0,anode electrode exp

electrode anode, cathode

RT E
j k

nF RT
 = − 
 

∈
 (20) 

The ohmic losses ( ohmη ) is as follows: 

ohm ohmjRη =  (21) 

where ohmR is the internal resistance depending on the conductivity and the thickness of the individual layers 
as shown below: 

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i
ohm

i i

R
τ
σ

=   (22) 

where τ  is the thickness (m), σ  is the electronic conductivity (Ω-1 m-1) and i  represents the anode, cathode 
and electrolyte. 

3.3 MCFC electrochemical model 

The open-circuit voltage ( OCVE ) of the MCFC is determined by Nernst equation: 

2 2

2 2 2

H O CO ,
0.5

H O CO ,

ln
2 2

a
OCV

c

p pG RT
E

F F p p p
 Δ

= − −   
 

 (23) 

where GΔ  is the Gibbs free energy. The subscribe a  and c  mean the anode and cathode, respectively. 
The operating voltage ( E ) can be expressed as: 

OCV lossE E η= −   (24) 
The internal voltage loss inside the MCFC based on a combined experimental and theoretical approach is 
defined as the equivalent global resistances which account for all type of losses. The equivalent global 
resistances are the resistances of the anode, cathode and electrolyte as follows (Morita et al., 2002): 

2 231.12 10 expirR RT
−  = ×  

 
 (25) 

2

3 0.5
H

23.7
2.04 10 expaR pRT

− − = ×  
 

 (26) 

2 2 2

9 0.75 0.5 6 1
O CO CO

132 67.1
3.28 10 exp 3.39 10 expcR p p yRT RT

− − − −   = × + ×   
   

 (27) 

where 
2CO

y is the mole fraction of carbon dioxide at the cathode. 

The total voltage losses can be written as Eq(28): 

( )loss ir a cR R R jη = + +  (28) 

4. Results and discussion 

Performance of the SOFC and MCFC integrated systems with different configurations is compared under the 
same operating condition. The performance is studied in term of the energy efficiency and the CO2 emission 
coefficient that tell the amount of generated power per 1 kilogram of methane feed and the amount of CO2 
released to produce 1 kW-h electricity, respectively. The recycle ratio (R) is considered for the system 
configuration A and B. Increasing in recycle ratio increases the concentration of carbon dioxide in cathode of 
MCFC and consequently reduces the cathode resistance. In case of the system configuration B and C, the 
reformed gas is divided into two streams which identified by feed ratio (F). High feed ratio means that the large 
amount of the reformed gas is fed into the SOFC. Feed ratio told the proper fuel feed in each cell. High feed 
ratio may enhance the efficiency but the CO2 may more release because of excess the CO2 requirement in 
MCFC. Thus, the performance of the fuel cell systems varies, depending on the recycle ratio and the feed 
ratio. The simulation results show that for the configuration A, the energy efficiency increases with increasing 
the recycle ratio, but the CEC shows an opposite trend (Figure 2(a)). The fuel cell system A is preferred to be 
operated at a higher recycle ratio. For the system configuration B, the energy efficiency and the CEC rapidly 
increase when increasing in the feed ratio (Figure 2(b) and (c)). An increase in the recycle ratio also increases 
the energy efficiency, but decreases the CEC. At high feed ratio, the recycle ratio has a slight effect of the 
system performance. For the system configuration C (Figure 2(d)), increasing the feed ratio rises the energy 
efficiency of the fuel cell system. The CEC initially decreases to its minimum and then increases with the 
increased feed ratio. Figures 2(e) and (f) compare the energy efficiency and CEC of different fuel cell systems 
at their optimal operating conditions. It clearly shows that the system configuration A provides the highest 
energy efficiency and lowest CEC.  

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Figure 2: Performance of the fuel cell system: (a) Configuration A, (b) and (c) Configuration B and (d) 
Configuration C, and comparison of the system energy efficiency (e) and CEC (f) 

5. Conclusions 

This study investigated and compared the performance of the SOFC and MCFC integrated systems with 
different configurations in terms of power generation and CO2 emission. The simulation results show that for 
the optimal fuel cell system, all the exhaust gases of the SOFC should be directly introduced to the MCFC and 
a recirculation of the exhaust gas from an after-burner to MCFC can enhance the energy efficiency and 
decrease the CO2 emission.  

Acknowledgements 

Support from The Thailand Research Fund (The Royal Golden Jubilee Ph.D. Program and The Institutional 
Research Grant) and Chulalongkorn University is gratefully acknowledged. 

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