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Optimal design of different reforming processesof the actual composition of bio-oil forhigh-temperature PEMFC systems

Suthida Authayanun a,*, Dang Saebea b, Yaneeporn Patcharavorachot c,Suttichai Assabumrungrat d, Amornchai Arpornwichanop e

a Department of Chemical Engineering, Faculty of Engineering, Srinakharinwirot University, Nakhon Nayok 26120,

Thailandb Department of Chemical Engineering, Faculty of Engineering, Burapha University, Chonburi 20131, Thailandc School of Chemical Engineering, Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang,

Bangkok 10520, Thailandd Center of Excellence in Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty

of Engineering, Chulalongkorn University, Bangkok 10330, Thailande Computational Process Engineering Research Unit, Department of Chemical Engineering, Faculty of Engineering,

Chulalongkorn University, Bangkok 10330, Thailand

a r t i c l e i n f o

Article history:

Received 24 July 2016

Received in revised form

18 October 2016

Accepted 21 October 2016

Available online 14 November 2016

Keywords:

Bio-oil

High-temperature PEMFC

Steam reforming

Autothermal reforming

Partial oxidation

* Corresponding author. Fax: þ66 3 732 2608E-mail address: suthidaa@g.swu.ac.th (S.

http://dx.doi.org/10.1016/j.ijhydene.2016.10.10360-3199/© 2016 Hydrogen Energy Publicati

a b s t r a c t

Hydrogen production from bio-oil, a by-product of the pyrolysis of palm empty fruit

bunches, using different reforming processes, i.e., steam reforming (SR), partial oxidation

(POX) and autothermal reforming (ATR), is theoretically investigated using the actual

composition of bio-oil. The effect of the reaction temperature, steam to carbon (S/C) ratio

and oxygen to carbon (O/C) ratio on the hydrogen production and coke formation of the

reformers is analysed. Favourable operating conditions to inhibit carbon formation, to

produce low CO concentrations and to achieve high hydrogen yields for the hydrogen

production processes coupled with a high-temperature water-gas shift reactor (HT-WGSR)

in a high-temperature proton exchange membrane fuel cell (PEMFC) system is also

investigated. The results show that an S/C ratio above two is preferred for the bio-oil steam

reformer to keep the CO concentration below the maximum allowable limit of the high-

temperature PEMFC. However, the CO concentration in the product gas from an HT-

WGSR integrated with an autothermal reformer and a partial oxidation reactor is lower

than the 5% limit at all temperatures (300e1000 �C), S/C ratios (1e2) and O/C ratios (0.3e1)

considered. The efficiency of different bio-oil reforming processes integrated with high-

temperature PEMFC systems is studied. The highest system efficiency is achieved from

the integrated system consisting of a bio-oil steam reformer, an HT-WGSR and a high-

temperature PEMFC with heat integration.

© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

.Authayanun).25ons LLC. Published by Els

evier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 1 9 7 7e1 9 8 81978

Introduction

Hydrogen is an energy carrier that is used to produce elec-

tricity via fuel cell applications. Most hydrogen is produced

from natural gas through a steam reforming process due to

the high performance and cost-effectiveness of this process.

Biomass is one of the most interesting fuels for hydrogen

production when environmental and availability aspects are

considered [1]. Sources of biomass can be plants, residues

from agriculture or forestry, and the organic components of

household and industrial wastes. The best-known thermo-

chemical processes for producing hydrogen from biomass are

the pyrolysis and gasification processes. For the biomass

gasification process, solid biomass is transformed to mainly

gas phase products in a gasifier in the presence of oxygen or

air. Alternatively, the biomass can be converted to hydrogen

by using pyrolysis followed by the reforming process [2]. In the

pyrolysis reactor, biomass is decomposed to liquid, gas and

char in the absence of oxygen. The liquid product is called bio-

oil and can be reformed to reformate gas in the reformer. The

low energy density and high transportation cost of biomass

are the main problems of hydrogen production from biomass

gasification [3]. Because bio-oil is easier and less expensive to

transport than biomass, the use of the biomass pyrolysis

process to produce bio-oil, which is in turn reformed to

hydrogen via reforming processes, is a promising way to

produce hydrogen from biomass from different locations [3,4].

Many works focus on the development of bio-oil reforming

processes using model compounds [5e7]. Gil et al. [8] studied

the sorption-enhanced steam reforming using acetic acid as

the model compound. They revealed that high steam to car-

bon ratio and low weight hourly space velocity (WHSV) in the

feed stream can enhance hydrogen production and suppress

methane, CO and CO2 formation in the product gas. High

hydrogen purities of up to 99.8% were obtained. Goicoechea

et al. [9] analysed steam reforming using acetic acid as the

model compound for bio-oil for a solid oxide fuel cell (SOFC)

system. They revealed that steam reforming of acetic acid is

thermodynamically feasible even at low steam to acetic acid

ratios. To obtain a high hydrogen yield for the SOFC, a steam

reformer should be operated at high temperatures and high

steam to acetic acid ratios. However, lower steam to acetic

acid ratios reduce the energy usage and improve the system

efficiency. Zhang et al. [10] developed catalysts for steam

reforming of acetic acid from bio-oil. They concluded that

Ni0:2Co0:8Mg6O7±d has more resistance to both carbon forma-

tion and oxidation of activemetals compared tomonometallic

catalysts. Most previousworks investigated only acetic acid as

the model compound. However, actual bio-oil has a wide va-

riety of components in addition to acetic acid. Many experi-

mental studies of reforming of actual bio-oil have already

been carried out [11e13]. Valle et al. [11] studied steam

reforming of the aqueous fraction of bio-oil with pyrolytic

lignin retention in a two-step reaction unit. For the first step, a

pyrolytic lignin is deposited by repolymerization of certain

bio-oil components, and the treated bio-oil is reformed in the

second step. These authors concluded that the catalyst

deactivation is low and the hydrogen yield and selectivity are

improved by using a Ni/La2O3ea Al2O3 catalyst. Additionally,

the steam reforming of raw bio-oil in a fluidized bed reactor

with CO2 capture was studied by Remiro et al. [12] CO2 was

effectively captured by using dolomite as an adsorbent in the

reforming reactor. The raw bio-oil was reformed at 600 �Cwithout adding water. In addition, Remiro et al. [13] investi-

gated the catalyst deactivation by coke deposition in the

steam reforming of the bio-oil aqueous fraction. They

concluded that the coke is gasified and Ni does not undergo

sintering at 700 �C.However, the theoretical study of hydrogen production

from the mixture of different oxygenated components of real

bio-oil is limited. Montero et al. [14] thermodynamically ana-

lysed the steam reforming of model compounds of real bio-oil

and ethanol. They used components in the bio-oil resulting

from the fast pyrolysis of pine sawdust as the model com-

pounds. However, the performance analysis and comparison

of different bio-oil reforming processes, steam reforming,

partial oxidation and autothermal reforming has not been

reported yet. In addition, the high molecular weight compo-

nents in bio-oil tend to be reformed to carbon, which causes

catalyst deactivation. Therefore, the study of reforming of

actual bio-oil with consideration of carbon formation is an

interesting topic.

The proton exchange membrane fuel cell (PEMFC) is a low-

temperature fuel cell that plays a leading role in trans-

portation, portable power and residential power applications.

PEMFCs can be classified into two types, the low-temperature

PEMFC (60e80 �C) and the high-temperature PEMFC

(100e200 �C). For pure hydrogen operation, high-temperature

PEMFCs show lower performance than low-temperature

PEMFCs due to low oxygen permeability and strong phos-

phate adsorption [15]. However, high-temperature operation

allows high-temperature PEMFCs to tolerate relatively high

levels of fuel impurities. To utilize reformate gas for proton

exchange membrane fuel cells, CO removal units are required

to reduce the CO content in the reformate gas. For conven-

tional PEMFCs, complex hydrogen purification units are typi-

cally needed to achieve CO concentrations below 10 ppm. For

high-temperature PEMFCs, only a water-gas shift reactor is

necessary due to the higher CO tolerance of this type of fuel

cell [16].

Recently, the integration of the hydrogen production pro-

cess with PEMFCs has received considerable attention.

Because of increased power demand and environmental

concerns, the development of a new sustainable feedstock for

hydrogen production is necessary. Chutichai et al. [17] studied

the integration of biomass gasification with the PEMFC sys-

tem. They proposed that the system efficiency is enhanced up

to 50% when waste heat is recovered to utilize in the system.

Guan et al. [18] investigated a PEMFC system fuelled by bio-gas

to cogenerate electricity and heat for a dairy farm and a biogas

plant. They concluded that the total efficiency of the PEMFC e

based combined heat and power system is 82%. In addition,

this system can reduce CO2 emissions by approximately 416

tons/year compared with a coal-fired combined heat and

power plant. Recently, our previous work [19] analysed the

performance of bio-ethanol steam reforming integrated with

a PEMFC system by considering the environmental impact.

The system integrating mixed bio-ethanol and methane

reforming with the PEMFC system achieved a higher system

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 en e r g y 4 2 ( 2 0 1 7 ) 1 9 7 7e1 9 8 8 1979

efficiency and a lower environmental impact than the system

integrating dehydrated bio-ethanol reforming with the PEMFC

because of the high energy consumption of the reformer,

preheating unit and bio-ethanol distillation unit in the system

using the dehydrated bio-ethanol reformer. In addition, our

previous work also investigated the performance of a high-

temperature PEMFC system fuelled by different non-

renewable and renewable fuels (i.e., methane, methanol,

ethanol, glycerol) [20]. The effect of CO poisoning on the high-

temperature PEMFC performance and the necessity of a

water-gas shift reactor as a CO removal unit were studied for

each fuel type. The results showed that a water-gas shift is

required to reduce the CO poisoning effect and to enhance the

system efficiency. However, only limited work on high-

temperature PEMFC systems that run on biomass-derived

bio-oil has been published. In addition, the operational

design of the reforming process coupled with the CO removal

process for HT-PEMFCs should be investigated.

In this study, a thermodynamic analysis of hydrogen pro-

duction from bio-oil, a by-product of pyrolysis of palm empty

fruit bunches, using different reforming processes, i.e., steam

reforming (SR), partial oxidation (POX) and autothermal

reforming (ATR), is investigated using the Aspen Plus simu-

lator and using the actual composition of the bio-oil. Simula-

tion studies are performed to determine the influence of key

operating parameters (e.g., reaction temperature, steam to

carbon (S/C) molar feed ratio, oxygen to carbon (O/C) molar

feed ratio) on the performance of a reformer in terms of

hydrogen production and coke formation. The integration of

the bio-oil reforming processes with the high-temperature

water-gas shift reactor (HT-WGSR) to produce hydrogen for a

high-temperature PEMFC was examined. The CO concentra-

tion at different reformer operating conditions is presented to

find conditions to produce a sufficiently low CO concentration

(<5%) for the HT-PEMFC. Subsequently, the optimal conditions

for bio-oil steam reforming, autothermal reforming and par-

tial oxidation processes integrated with an HT-WGSR to

inhibit carbon formation, to achieve a low CO concentration

and to maximize the hydrogen fraction are presented. In

addition, the efficiency of the integrated high-temperature

PEMFC systems at different reforming processes is studied

and compared.

Process description

The integrated high-temperature PEMFC systems examined in

this work are shown in Fig. 1. Each system consists of a fuel

processing process and a high-temperature PEMFC. Steam

reforming, autothermal reforming and partial oxidation are

the hydrogen production processes and the high-temperature

water-gas shift reactor is only used as a hydrogen purification

process to decrease the CO concentration below 5%.

Fuel processing process

In this work, the composition of actual bio-oil produced from

empty palm fruit bunch fast pyrolysis is used [21]. The

composition of the bio-oil is shown in Table 1. The product

compositions of the reformer and the water-gas shift reactor

at equilibrium are calculated by minimization of the Gibbs

free energy, as shown in Eq. (1):

minni

ðGtÞT;P ¼XCi¼1

niGi ¼XCi¼1

ni

G

�i þ RT ln

f if �i

!(1)

where C is the total number of components in the reaction

system, and ni is the amount of each gaseous component.

Because of the conservation of atomic species, ni must satisfy

the element balance in Eq. (2):

Xi

niaik �Ak ¼ 0 ðk ¼ 1; 2; :::;wÞ (2)

where aik is the number of atoms of element k in component i,

Ak is the total number of atoms of element k in the reaction

mixture, and w is the total number of elements.

The reactions of oxygenates due to steam reforming, par-

tial oxidation and autothermal reforming, along with the

possible side reaction, are presented in Table 2. The compo-

nents in the steam reforming system are the mixture of the

actual components of bio-oil (see Table 1), CH4, H2, CO, CO2, C

(Graphite), C2H2, C2H4 and H2O. The O2 component is added to

the reactive system for the partial oxidation and autothermal

reforming processes. The structure and properties of the

components in bio-oil (i.e., 1-hydroxy-2-butanone, 2(5H)-fur-

anone, 2-cyclopenten-1-one, 2-hydroxy-3-methyl, 2-

methoxy-(guaiacol), 2,6-dimethyoxy-(syringol), and Levoglu-

cosan) are input to the Aspen Plus program since the ther-

modynamic properties of these components are not available

in conventional thermodynamic databases.

The Rgibbs reactor module in Aspen Plus simulator is used

to represent the steam reformer, the autothermal reformer,

the partial oxidation reactor and the water-gas shift reactors.

Methane is treated as an inert gas in the water-gas shift

reactor. The equation of state used in the calculation is based

on the SoaveeRedlicheKwong equation [9]. The HT-WGSR is

operated at a temperature of 350 �C, and the ratio of steam to

CO in the feed stream is greater than two. Additional steam is

added if the steam to CO ratio of the reforming gas is lower

than two. Table 3 shows the validation of the steam reforming

of the acetic acid model compound using a steam reformer

temperature of 800 �C, an S/C ratio of 6 and atmospheric

pressure. The simulation results agree with the experimental

data reported by Hu and Lu [22].

High-temperature PEMFCs

The high-temperature PEMFC model based on electro-

chemical reactions is coded in FORTRAN in the Aspen Plus

simulator to predict the relation between voltage and current

density. An ideal separator, Sep, is applied for the anode to

separate the hydrogen from the reformate gas, and the cath-

ode is modelled as an Rgibbs reactor. The cell temperature of

the high-temperature PEMFC is specified as 175 �C. A PBI-

doped phosphoric acid membrane is used as the electrolyte

for the high-temperature PEMFC. The actual cell potentials or

operating voltage of a fuel cell (Ecell) is always smaller than the

reversible cell potential (Er) due to irreversible losses. Ecell can

be calculated from the activation loss at the anode ðhact;aÞ, theactivation loss at the cathode ðhact;cÞ and the ohmic loss

ðhohmicÞ as shown in Eq. (3).

Fig. 1 e The integrated high-temperature PEMFC systems.

Table 1 e The composition of bio-oil from empty palmfruit bunch fast pyrolysis [21].

Components Chemicalformula

Composition(wt%)

Acetic acid C2H4O2 56.73

Propanoic acid (acetol) C3H6O2 1.09

1-hydroxy-2-butanone C4H8O2 3.49

2(5H)-furanone C4H4O2 0.33

2-cyclopenten-1-one,

2-hydroxy-3-methyl

C6H8O2 7.90

Phenol C6H6O 23.97

Phenol, 2-methoxy-(guaiacol) C7H8O2 1.70

Phenol, 2,6-dimethyoxy-(syringol) C8H10O3 2.90

Levoglucosan C6H10O5 1.88

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 1 9 7 7e1 9 8 81980

Ecell ¼ Er � hact;a � hact;c � hohmic (3)

The reversible cell potential is described by the Nernst

equation.

Er¼��DHT

nF� TDST

nF

�þ RT

nFln

"ðRTÞ1:5CPt;aC0:5

Pt;c

aH2O

#(4)

where aH2O is the water activity defined by the ratio of water

partial pressure to its saturation pressure ðPsatH2O

Þ, which is

given in Eq. (5). CPt,a and CPt,c are the concentrations of

hydrogen and oxygen at the anode and cathode catalyst sur-

face, which are calculated by the StefaneMaxwell equation

and Fick's law, as shown in Table 4. Deffij is calculated using the

SlatteryeBird correlation [23] and corrected to account for the

porosity/tortuosity effects using the Bruggeman correlation

[24].

Table 2 e Reactions involved in the different hydrogenproduction processes.

Steam reforming

CnHmOk þ ðn� kÞH2O/nCOþ ðnþm=2� kÞH2

CnHmOk þ ð2n� kÞH2O/nCO2 þ ð2nþm=2� kÞH2

Partial oxidation

CnHmOk þ ðn=2� kÞO2/nCOþ ðm=2ÞH2

CnHmOk þ ðn� kÞO2/nCO2 þ ðm=2ÞH2

Autothermal reforming

CnHmOk þ 12 ðn=2� kÞO2 þ 1

2 ðn� kÞH2O/nCOþ 12 ðnþm� kÞH2

CnHmOk þ 12 ðn� kÞO2 þ 1

2 ð2n� kÞH2O/nCO2 þ 12 ð2nþm� kÞH2

Additional side reactions for hydrogen production processes

Wateregas shift reaction

CO þ H2O4 CO2 þ H2

Methanation reaction

COþ 3H2/CH4 þH2O

Carbon formation reactions

2CO4CO2 þ C

CH442H2 þ C

COþH24H2Oþ C

Table 3 e The validation of the acetic acid steamreforming process at a reformer temperature of 800 �C, S/C ratio of 6 and atmospheric pressure.

Component % Conversion or yield % Error

Simulation Experimental [22]

Acetic acid 100 100 0.00

H2 86 88 2.27

CO 12.7 13 2.31

CO2 80.1 82 2.32

CH4 0 0 0.00

Table 4 e Diffusion models used in the simulation of the high

Diffusion models

Stefan Maxwell models

Fick' law models

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 en e r g y 4 2 ( 2 0 1 7 ) 1 9 7 7e1 9 8 8 1981

PsatH2O

¼ �142:07682T4 � 171026:12676T3 þ 78013638:11584T2

� 15953375633:8471Tþ 1231888491801:45�*10�10 (5)

Activation losses are governed by the ButlereVolmer

equation (Eqs. (6) and (7)).

ia ¼ i0;a

�exp

��aRd;aFRT

�hact;a

��� exp

��aOx;aFRT

�hact;a

���(6)

ic ¼ i0;c

�exp

��aRd;cFRT

�hact;c

��� exp

��aOx;cFRT

�hact;c

���(7)

where i is the current density and a is the transfer coefficient.

i0 is the exchange current density, which can be calculated

from Eqs. (8) and (9) for the anode and cathode, respectively.

i0;a ¼ iref0;aac;aLc;a

CPt;a

CrefPt;a

!g

exp

�� Ec;a

RT

�1� T

Tref ;a

��(8)

i0;c ¼ iref0;cac;cLc;c

CPt;c

CrefPt;c

!g

exp

�� Ec;c

RT

�1� T

Tref ;c

��(9)

Assuming aRd ¼ aOx ¼ a, the hyperbolic sine function can be

substituted in Eqs. (6) and (7), yielding the following

relationship:

hact ¼RTaF

sinh�1

�i2i0

�(10)

The effect of CO poisoning is included in the anode acti-

vation loss model of the high-temperature PEMFCs because

hydrogen-rich gas from the fuel processor is used at the

anode. The anode activation loss can be calculated from the

exchange current density of hydrogen and the CO coverage

-temperature PEMFC.

Equations

At the anode

dXCO2dz ¼ RT

P XCO2

0@ NH2 ;g

DeffH2 ;CO2

1A

dXH2O

dz ¼ RTP XH2O

0@ NH2 ;g

DeffH2 ;H2O

1A

dXCOdz ¼ RT

P XCO

0@ NH2 ;g

DeffH2 ;CO

1A

dXCH4dz ¼ RT

P XCH4

0@ NH2 ;g

DeffH2 ;CH4

1A

XH2 ¼ 1� XCO2 � XH2O � XCO � XCH4

At the cathode

dXN2dz ¼ RT

P XN2

0@ NO2

DeffN2 ;O2

þ NH2O;g

DeffN2 ;H2O

1A

dXH2O

dz ¼ RTP

24XH2O

0@ NO2

DeffO2 ;H2O

1A� NH2O

0@ XO2

DeffO2 ;H2O

þ XN2

DeffN2 ;H2O

1A35

XO2 ¼ 1� XN2 � XH2O

At the anodeNH2

SPt�anode¼ �DH2

ðCPt;a�CH2 ðdissolveÞ Þdanode

CH2ðdissolveÞ ¼ CdissolvedH2

$XH2 $P

At the cathodeNO2

SPt�cathode¼ �DO2

ðCPt;c�CO2 ðdissolveÞ Þdcathode

CO2ðdissolveÞ ¼ CdissolvedO2

$XO2 $P

Table 5 e Parameters used for the high-temperaturePEMFC [15].

Parameters Value

Cell temperature, T (�C) 175

Operating pressure at anode, Pa (atm) 1

Operating pressure at cathode, Pc (atm) 1

Stoichiometric ratio at anode, Sa 1.25

Stoichiometric ratio at cathode, Sc 2

Membrane thickness, lm (m) 4 � 10�5

GDL thickness, z (m) 0.0002

Anode film thickness, danode (m) 2.5 � 10�9

Cathode film thickness,dcathode (m) 1.48 � 10�9

Anode activation energy, Ec,a (J mole�1 K�1) 16,900

Cathode activation energy, Ec,c (J mole�1 K�1) 72,400

Anode reference cell temperature, Tref,a (�C) 160

Cathode reference cell temperature, Tref,c (�C) 100

Anode catalyst surface area, ac,a (m2 g�1) 64

Cathode catalyst surface area, ac,c (m2 g�1) 32.25

Anode catalyst loading, Lc,a (mg cm�2) 0.2

Cathode catalyst loading, Lc,c (mg cm�2) 0.4

Transfer coefficient at anode, aa 0.5

Transfer coefficient at cathode, ac 0.75

Reaction order at anode, ga 1

Reaction order at cathode, gc 1.375

Anode reference concentration, CrefPt;a (mol cm�3) 0.0002

Cathode reference concentration, CrefPt;c (mol cm�3) 0.0004

Temperature (oC)

300 400 500 600 700 800

H2

mol

ar fl

ow ra

te (m

ol/s)

0

2

4

6

8

S/C = 1S/C = 1.5S/C = 2S/C = 2.5S/C = 3

S/C = 0.5S/C = 1S/C = 1.5

Inhibited carbon formation

Carbon formation

Fig. 2 e Effect of the reformer temperature and S/C ratio on

the hydrogen molar flow rate of the steam reforming

process.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 1 9 7 7e1 9 8 81982

(qCO) [25] as shown in Eq. (11). The bridge model of CO

adsorption on Pt and the anode activation loss of HT-PEMFC is

represented by Eq. (12).

hact;a ¼ RTaF

sinh�1

i

2i0ð1� qCOÞ2!

(11)

qCO ¼ a*ln½CO�½H2� þ b*lnðiÞ*ln ½CO�

½H2� þ c (12)

where

a ¼ �0:00012784�T2 þ 0:11717499�T� 26:62908873b ¼ 0:0001416�T2 � 0:12813608�T þ 28:852463626c ¼ �0:00034886�T2 þ 0:31596903�T� 70:11693333

The ohmic loss for a high-temperature PEMFC is described

as follows:

hohmic ¼ Rmemi ¼�sm

lm

�i (13)

where Rmem is the membrane resistance, sm is the proton

conductivity of the membrane and lm is the membrane

thickness.

Table 6 e The models of efficiency of high-temperature PEMFC

Efficiency

Reformer efficiency ðhRÞFuel processor efficiency ðhFP ÞFuel cell efficiency ðhFCÞSystem efficiency without heat integration ðhSÞSystem efficiency with heat integration ðhSCÞ

The proton conductivity as a function of temperature and

relative humidity [26] can be calculated from Eqs. (14)e(16).

sm ¼ ATexp

� �BRðTÞ

�(14)

A ¼ exp��ka1RH

3�þ �ka

2RH2�þ �ka

3RH�þ ka

0

�(15)

B ¼�kb1RH

3þ�kb2RH

2þ�kb3RH

þ kb

0 (16)

The parameters used for the simulation of high-

temperature PEMFCs are shown in Table 5. In addition, the

detailed calculations of reformer efficiency, fuel processor

efficiency, fuel cell efficiency and system efficiency can be

found in Table 6. To calculate the system efficiency with heat

integration, heat recovered from the high-temperature PEMFC

is used to preheat the reactants for the fuel processing

processes.

Results and discussion

Reforming processes

For the steam reforming process, the effect of the reformer

temperature and S/C ratio on the hydrogen molar flow rate is

shown in Fig. 2. The dashed lines showoperating conditions to

systems.

Equations

hR ¼ LHVH2$ð _mH2

ÞLHVfuel $ð _mfuelÞþQT

QT ¼ QR1 þ QR2 �QR3

hFP ¼ LHVH2$ð _mH2

ÞLHVfuel $ð _mfuelÞþQT

QT ¼ QR1 þQR2 �QR3 þQP1

hFC ¼ PFCð _mH2

Þ$LHVH2

hS ¼ PFCLHVfuel $ð _mfuelÞþQT

QT ¼ QR1 þQR2 � QR3 þQP1 �QP2 þQF1

hSC ¼ PFCLHVfuel $ð _mfuelÞþQT

QT ¼ QR1 þ QR2 �QR3 þQP1 �QP2 þ QF1 �QF2

Temperature (oC)

300 400 500 600 700 800 900 1000

H2

mol

ar fl

ow ra

te (m

ol/s)

0

1

2

3

4

O/C = 0.6O/C = 0.7O/C = 0.8

O/C = 0.1O/C = 0.2O/C = 0.3O/C = 0.4O/C = 0.5O/C = 0.6 O/C = 0.7O/C = 0.8

Carbon formation Inhibited carbon formationO/C = 0.3O/C = 0.4O/C = 0.5

Fig. 3 e Effect of the reformer temperature and O/C ratio on

the hydrogen molar flow rate of the partial oxidation

process.

Fig. 4 e Effect of the reformer temperature and O/C ratio on the

process at different S/C ratios: (a) S/C ¼ 0.5, (b) S/C ¼ 1, (c) S/C ¼

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 en e r g y 4 2 ( 2 0 1 7 ) 1 9 7 7e1 9 8 8 1983

be avoided because carbon formation occurs, whereas the

solid lines show the hydrogen production rate where carbon

formation does not occur. The steam reformer should be

operated at temperatures of 650e700 �C to maximize the

hydrogen molar flow rate. Carbon formation is unfavourable

at S/C ratios higher than two at all reformer temperatures

considered. Compared to the steam reforming of the acetic

acid model compound, higher S/C ratios are required for bio-

oil steam reformer to prevent carbon formation due to the

higher molecular weight compounds contained in actual bio-

oil [27]. In addition, the simulation predictions of carbon for-

mation for bio-oil steam reforming are in agreement with the

results of Montero et al. [14]. Although carbon formation can

be avoided by adding excess steam into the reformer, it still

occurs at high temperatures, as observed from experimental

data [13]. This is because the assumed graphite carbon in the

simulation is not the complete representation of the carbon

that is deposited, and it is a very complex task to thermody-

namically model carbon deposition [14,28]. However, Hu and

Lu [29], who experimentally investigated bio-oil steam

hydrogen molar flow rate of the autothermal reforming

1.5 and S/C ¼ 2.

Fig. 5 e Effect of the reformer temperature and S/C ratio on

the performance of the bio-oil steam reformer integrated

with HT-WGSR at the carbon formation-inhibiting

condition: (a) %CO (dry basis) and (b) Hydrogen mole

fraction (dry basis).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 1 9 7 7e1 9 8 81984

reforming, partial oxidation and oxidative steam reforming

with CO2 utilization, proposed that high temperature has a

positive effect on reducing coke formation. Therefore, further

study and characterization of carbon is necessary, and it is an

interesting topic for furthering the understanding of carbon

formation in real systems [30]. In addition, it is noted that a

high S/C ratio and high temperature are required to enhance

hydrogen production and to prevent carbon formation.

Furthermore, the effect of reformer temperature and the O/

C ratio on the hydrogen molar flow rate for the partial oxida-

tion of actual bio-oil is presented in Fig. 3. The partial oxidation

reactor is operated at atmosphere pressure and isothermal

conditions. Additionally, the operating conditions represented

by the dashed line and by the solid line show the hydrogen

production with carbon formation and with inhibited carbon

formation, respectively. According to the simulation results,

carbon is formed at all O/C ratios (0.1e0.8) studied. At high O/C

ratios (O/C ratio � 0.3), the dashed lines become solid lines at

higher temperatures. However, the hydrogen content de-

creases with increasing O/C ratio. From Fig. 3, the hydrogen

molar flow rate increases with increasing temperature until

reaching a maximum. Beyond that point, further increase in

temperature slightly reduces the hydrogen production rate.

The maximum hydrogen molar flow rate is observed at tem-

peratures greater than 900 �C and O/C ratios of 0.1 and 0.2, but

carbon formation is promoted at these conditions. Therefore,

the optimal condition with inhibited carbon formation for

partial oxidation of actual bio-oil is an O/C ratio of 0.3 and a

temperature above 800 �C. Compared to the steam reforming

process, the partial oxidation process provides a lower

hydrogen content and needs to be operated at a higher

temperature.

Next, the performance of autothermal reforming of actual

bio-oil for hydrogen production is thermodynamically ana-

lysed under atmospheric pressure and at isothermal condi-

tions. The effect of the operating parameters of autothermal

reforming of actual bio-oil on the hydrogen molar flow rate is

shown in Fig. 4. In this process, both steam and oxygen are

used as agents to reform bio-oil into reformate gas. In Fig. 4(a),

which shows the performance of autothermal reforming of

the actual bio-oil composition at an S/C ratio of 0.5, the dashed

lines appear at all operating O/C ratios. This means carbon

formation is favourable at these conditions. However, the

maximum hydrogen molar flow rate can be achieved at a

reformer temperature of 800 �C and an O/C ratio of one with

inhibited carbon formation.

In addition, the results further show that no dashed lines

(meaning carbon formation did not occur) are found at high O/

C ratios when the S/C ratio increases to 1 and 1.5, as shown in

Fig. 4(b) and (c), respectively. For an autothermal reformer

operated at an S/C ratio of one in Fig. 4(b), carbon did not

appear in the products of the autothermal reformer at all

operating temperatures when the O/C ratio was greater than

0.6. In addition, the O/C ratio needed to prevent carbon for-

mation at all reformer temperatures should be greater than

0.3 when an S/C ratio of 1.5 is specified, as shown in Fig. 4(c).

For an autothermal reformer operated at an S/C ratio of two,

as shown in Fig. 4(b), carbon formation is avoided at all

operating temperatures and O/C ratios studied. The hydrogen

production of the autothermal reformer of actual bio-oil is

enhanced at high S/C ratios, reformer temperatures of

700e800 �C and anO/C ratio of 0.1. However, it should be noted

that operation at higher O/C ratios can reduce the require-

ment of external heat for the reformer.

Reforming processes integrated with a CO removal unit

The CO and hydrogen concentrations at different operating

parameters of a steam reformer, a partial oxidation reactor

and an autothermal reformer are investigated when the

hydrogen production processes are integrated with a high-

temperature water-gas shift reactor (HT-WGSR). It is noted

that the optimal operating conditions for hydrogen produc-

tion processes should not only maximize hydrogen concen-

tration but also achieve a CO fraction lower than 5%.

Therefore, the CO concentration in the product gases at

various operating conditions is first studied and then the ef-

fect of operating conditions on the hydrogen fraction while

avoiding carbon formation and achieving CO fractions <5% is

investigated. For the bio-oil steam reformer integratedwith an

Fig. 6 e Effect of the reformer temperature and O/C ratio on

the performance of the bio-oil partial oxidation reactor

integrated with HT-WGSR at the carbon formation-

inhibiting condition: (a) %CO (dry basis) and (b) Hydrogen

mole fraction (dry basis).

(a)

(b)

Temperature (oC)

300 400 500 600 700 800

%C

O (d

ry b

asis)

0

1

2

3

4

5

6

O/C = 0.1O/C = 0.2O/C = 0.3

CO limitation

Inhibited carbon formation

Temperature (oC)

300 400 500 600 700 800

H2 m

ole

frac

tion

(dry

bas

is)

00.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

O/C = 0.1O/C = 0.2O/C = 0.3

Inhibited carbon formation and CO < 5%

Fig. 7 e Effect of the reformer temperature and O/C ratio on

the performance of the bio-oil autothermal reformer

integrated with HT-WGSR at the carbon formation-

inhibiting condition: (a) %CO (dry basis) and (b) Hydrogen

mole fraction (dry basis). Dotted line e S/C ¼ 1; Dashed line

e S/C ¼ 1.5; Solid line e S/C ¼ 2.

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 en e r g y 4 2 ( 2 0 1 7 ) 1 9 7 7e1 9 8 8 1985

HT-WGSR, the CO fraction increases with increasing temper-

ature andwith decreasing S/C ratio, as shown in Fig. 5(a). At S/

C ratios of 1 and 1.5, the CO concentration is greater than 5% at

almost all reformer temperatures studied. Therefore, the

steam reformer should be operated at an S/C ratio �2 to keep

the CO concentration below the maximum allowable limit of

an HT-PEMFC. To find the optimal condition that maximizes

hydrogen concentration, the effect of the S/C ratio and the

reformer temperature on the hydrogenmolar fraction from an

HT-WGSR is studied at operating conditions that inhibit car-

bon formation and produced a CO concentration below 5%.

The simulation results are presented in Fig. 5(b). The highest

hydrogen concentration is obtained at reformer temperatures

of 700e800 �C and S/C ratios of 2 or higher. However, it should

be noted that operation at high temperature (above 700 �C)causes catalyst deactivation by sintering [2].

The partial oxidation reactor integrated with an HT-WGSR

is examined next. The effect of the reformer temperature and

the O/C ratio on CO fraction and hydrogen fraction in the

product gas from theHT-WGSR is presented in Fig. 6(a) and (b).

Fig. 6(a) shows that increasing the temperature from 300 to

600 �C increases the CO concentration. Further increase in

temperature results in only small changes of the CO fraction.

When the O/C ratio is lower than 0.9, the CO fraction increases

with increasing O/C ratio. However, the CO concentration

decreases when the O/C ratio increases from 0.9 to 1. The

lowest CO concentration for a partial oxidation reactor inte-

grated with an HT-WGSR is obtained at an O/C ratio of 1. This

is because under conditions with a high amount of oxygen,

fuel is converted to CO2 instead of CO. The reactions for partial

oxidation with different products (CO and CO2) are shown in

Table 2. Fig. 6(b) shows that a low O/C ratio is preferred to

enhance hydrogen production. Increasing temperature im-

proves the hydrogen concentration until it reaches an optimal

value for each O/C ratio. In addition, it can be concluded that

the CO fraction is below the limitation of the HT-PEMFC at all

of the operating conditions studied.

The effect of temperature, the S/C ratio and the O/C ratio of

an autothermal reformer integrated with an HT-WGSR on the

fuel processor performance is shown in Fig. 7. These operating

conditions have a significant effect on the CO and hydrogen

Table 7 e The optimal operating conditions and productcompositions of the different reformers for high-temperature PEMFC systems at inhibited carbonformation and CO < 5%.

Steamreformer

Autothermalreformer

Partialoxidation reactor

Temperature (�C) 700 700 850

S/C 2 1 e

O/C e 0.1 0.3

Hydrogen fraction 0.67 0.64 0.61

%CO 4.15 2.82 2.58

Fig. 8 e The efficiency of the different high-temperature

PEMFC systems.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 1 9 7 7e1 9 8 81986

fractions, as shown in Fig. 7(a) and (b), respectively. At an S/C

ratio of two, increasing the O/C ratio decreases the CO con-

centration. However, the CO fraction decreases with

increasing O/C ratio at an S/C ratio lower than two. This is

because the CO is still high at low S/C ratios, as seen in

Fig. 5(a); therefore, the addition of oxygen has a significant

effect on the CO concentration and can considerably reduce

the CO content in the product gas. From Fig. 7(b), a high S/C

ratio and a low O/C ratio are preferred for hydrogen produc-

tion with a high hydrogen fraction (dry basis). The optimal

operating temperature is 700 �C or higher. To apply the

product gas for an HT-PEMFC, the optimal operating param-

eters of the steam reformer, autothermal reformer and partial

oxidation reactor for an HT-PEMFC system to inhibit carbon

formation, obtain a CO fraction <5%, and maximize the

hydrogen fraction are shown in Table 7.

HT-PEMFC systems

The efficiency of the system and the efficiency of each unit of

the different hydrogen production processes and HT-WGSR

integrated with an HT-PEMFC are also studied, as shown in

Fig. 8. The steam reforming process achieves the highest

reformer efficiency (66%) and the partial oxidation process

shows the lowest reformer efficiency (34%), whereas the

reformer efficiency of the autothermal reforming process is

approximately 51%. However, the fuel processor efficiency

(efficiency of hydrogen production processes integrated with

HT-WGSR) is enhanced from the reformer efficiency because

additional hydrogen is produced from the HT-WGSR. The fuel

processor efficiencies of the steam reformer, autothermal

reformer and partial oxidation reactor are 75%, 71% and 66%,

respectively. However, the fuel cell efficiency of each

reforming process is quite similar due to the comparable

hydrogen fraction, as seen in Table 7. In should be noted that

the oxidant used in the autothermal reformer and the partial

oxidation reactor is oxygen, and thus no nitrogen dilution

effect occurs. The system efficiency improves when heat re-

covery from the HT-PEMFC is applied to the hydrogen pro-

duction processes. The system using a steam reformer, HT-

WGSR and HT-PEMFC with heat integration achieves the

highest system efficiency (41%).

Conclusions

This study is focused on hydrogen production for HT-PEMFC

systems to inhibit carbon formation and achieve a low CO

concentration (CO < 5%). The actual components of bio-oil

from fast pyrolysis of empty palm fruit bunch are used as

the fuel for hydrogen production from steam reforming, par-

tial oxidation and autothermal reforming processes. High re-

action temperatures and high S/C ratios enhances hydrogen

production, but hydrogen production decreases with

increasing O/C molar ratio. However, reformer operation at

high S/C molar ratios, high O/C molar ratios and high tem-

peratures is required to prevent coke formation. It should be

noted that graphite carbon does not fully represent carbon

deposition and that further study of this critical issue is

necessary. For a steam reformer integrated with HT-WGSR,

the steam reformer should be operated at S/C � 2 to keep

the CO fraction below 5%, which is the requirement for HT-

PEMFC. However, the CO concentration in the product gas

fromHT-WGSR is lower than 5% at all temperatures, S/C ratios

and O/C ratios considered for the autothermal reformer and

the partial oxidation reactor. In addition, the optimal oper-

ating conditions of the steam reformer, autothermal reformer

and partial oxidation reactor for the HT-PEMFC system to

maximize the hydrogen concentration, to inhibit carbon for-

mation, and to maintain the CO fraction <5% are presented as

well. When the different reforming options are compared, the

HT-PEMFC system based on the steam reforming process

achieved the highest system efficiency (41%), followed by the

HT-PEMFC system based on autothermal reforming (38%) and

the partial oxidation processes (35%).

Acknowledgements

Support from the Srinakharinwirot University and the Chu-

lalongkorn Academic Advancement into its 2nd Century

Project and the Thailand Research Fund (DPG5880003), is

gratefully acknowledged.

Nomenclatures

ac Catalyst surface area, m2 g�1

aH2O Water activity

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 en e r g y 4 2 ( 2 0 1 7 ) 1 9 7 7e1 9 8 8 1987

Cdissolve Equilibrium concentration, mol cm�3

CPt Concentration on the catalyst surface, mol cm�3

CrefPt Reference concentration on the catalyst surface,

mol cm�3

Deffij Binary diffusion coefficient, m2 s�1

Ec Activation energy, J mol�1 K�1

Ecell Cell voltage, V

Er Reversible cell potential, V

F Faraday constant, 96,485 C mol�1

H Enthalpy, J mol�1

i Current density, A m�2

i0 Exchange current density, A m�2

iref0 Reference exchange current density, A m�2

Lc Catalyst loading, mg cm�2

lm Membrane thickness, m

m Molar flow rate, mol s�1

LHV Lower heating value, kJ mol�1

N Molar flux, mol s�1 m�2

P Pressure, atm

PFC Power output of fuel cell, W

Q Heat flow, J s�1

QT Total heat required for the hydrogen production

process, J s�1

R Gas constant (¼8.314), J mol�1 K�1

SPt Real platinum surface area

T Cell temperature, K

X Mole fraction

Greek letters

d Average film thickness, m

a Transfer coefficient

g Reaction order

qCO CO coverage

qH H2 coverage

sm Proton conductivity, S cm�1

hFP Fuel processor efficiency

hact Activation loss, V

hohmic Ohmic loss, V

hS System efficiency without heat integration

hSC System efficiency with heat integration

hR Reformer efficiency

hFC Fuel cell efficiency

Subscripts and superscripts

a Anode

c Cathode

m Membrane

i,j Components “i” and “j”

in Inlet stream

out Outlet stream

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