Effect of operating parameters on performance of an integrated biomass gasifier, solid oxide fuel cells and micro gas turbine system

b i o m a s s a n d b i o e n e r g y 7 5 ( 2 0 1 5 ) 3 5 e4 5

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Effect of operating parameters on performance of
an integrated biomass gasifier, solid oxide fuel cells
and micro gas turbine system
Junxi Jia a,*, Abuliti Abudula b, Liming Wei c, Baozhi Sun a, Yue Shi a
a

College of Power and Energy Engineering, Harbin Engineering University, Harbin 150001, China
North Japan Research Institute for Sustainable Energy, Hirosaki University, Aomori 030-0813, Japan
c
School of Electric and Electronic Information Engineering, Jilin Jianzhu University, Changchun 130118, China
b

article info

abstract

Article history:

An integrated power system of biomass gasification with solid oxide fuel cells (SOFC) and

Received 13 October 2014

micro gas turbine has been investigated by thermodynamic model. A zero-dimensional

Received in revised form

electrochemical model of SOFC and one-dimensional chemical kinetics model of down-

5 February 2015

draft biomass gasifier have been developed to analyze overall performance of the power

Accepted 6 February 2015

system. Effects of various parameters such as moisture content in biomass, equivalence

Available online

ratio and mass flow rate of dry biomass on the overall performance of system have been
studied by energy analysis.

Keywords:

It is found that char in the biomass tends to be converted with decreasing of moisture

Biomass gasification

content and increasing of equivalence ratio due to higher temperature in reduction zone of

Solid oxide fuel cell

gasifier. Electric and combined heat and power efficiencies of the power system increase

Chemical equilibrium

with decreasing of moisture content and increasing of equivalence ratio, the electrical

Kinetics model

efficiency of this system could reach a level of approximately 56%.Regarding entire

Combined heat and power

conversion of char in gasifier and acceptable electrical efficiency above 45%, operating
condition in this study is suggested to be in the range of moisture content less than 0.2,
equivalence ratio more than 0.46 and mass flow rate of biomass less than 20 kg hÀ1.
© 2015 Elsevier Ltd. All rights reserved.

1.

Introduction

Biomass is supposed to be one of the most common renewable
sources used for power generation [1]. Biomass gasification
(BG) technology has been used to produce syngas and
electricity, from laboratory scale test to some demonstration
scale plants. Although low energy density and seasonal
availability of biomass lead to both the high transport cost and
high capital cost of biomass plants, it has potential of being

* Corresponding author.
E-mail address: (J. Jia).
/>0961-9534/© 2015 Elsevier Ltd. All rights reserved.

commercialized to produce hydrogen in the future [2]. Solid

oxide fuel cell (SOFC) is considered one of the most important
energy technologies for its high efficiency and low environmental impact. It is ideal for syngas conversion due to its high
operation temperature [3e5].
Integration of BG with SOFC has received more attention as
a potential substitute for fossil fuels in electric power production since it combines the merits of renewable energy
sources and hydrogen energy systems.


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b i o m a s s a n d b i o e n e r g y 7 5 ( 2 0 1 5 ) 3 5 e4 5

Thermodynamic analysis of BG and SOFC hybrid systems
have been reported by many researchers [6e14]. These studies
mainly focus on effect of operating conditions on overall
performance of the power systems.
Athanasiou et al. [8] and Cordiner et al. [9] investigated an
integrated process of biomass gasification and solid oxide fuel
cells system, the overall electrical efficiency could reach very
high level of more than 40%.Fryda et al. [10] assessed the
combination of BG with SOFCs and micro gas turbine (MGT).
Their results show that an electrical efficiency of 40.6% could
be achieved at elevated pressures. A hybrid plant consisting of
gasification system, solid oxide fuel cells and organic Rankine
cycle has been presented by Pierobon et al. [11]. The results
show that efficiencies over 54% can be achieved. Colpan et al.
[12] studied the effect of gasification agent (air, enriched oxygen and steam) on the performance of an integrated SOFC
and BG system. The results show that using steam as the
gasification agent yields the highest electrical efficiency of
41.8%.Rokni et al. [13] reported a hybrid plant producing

combined heat and power (CHP) from BG, SOFC and a MGT. An
electrical efficiency of 58.2% has been reported resulting from
optimization efforts.
Recently, Campitelli et al. [14] have invested the effect of
operating conditions on BG-SOFC systems performance. The
influence of H2 utilization of SOFC and moisture content in
biomass have been analyzed in details. In their work, a zerodimensional chemical equilibrium model was used in
gasifier. The authors did not take into account any char conversion in the reduction zone of gasifier.
Most of gasification models adopted to analyze the performance of BG, SOFC, and GT system mentioned above [6e14]
are based on thermodynamic equilibrium as those reported in
Refs. [15e18]. These equilibrium models are developed by the
thermodynamic parameters based on minimization of Gibbs
free energy. Although these pure equilibrium models are
relatively easy to be applied with fast convergence, they have
certain limitations such as considering sufficient residence

time, high reaction temperature, and fast reaction rates. The
dying, pyrolysis and oxidant process is assumed to be lumped
together in a single reaction. The gas compositions and temperature remains essential uniform in gasifier rather than
variable with the height of the gasifier. All the char is assumed
to be completely consumed before leaving the gasifier, which
could not take place in actual gasification process.
Since few of chemical kinetic model of gasifier is available
for analysis of an integrated BG, SOFC, and GT system, in this
paper kinetics model of downdraft biomass gasifier is presented in order to overcome the limitations of the equilibrium
model. The gas composition, reaction temperature, and unreacted char are predicted along height of the reduction
zone. Effect of process parameters, such as moisture content,
equivalence ratio and mass flow rate of dry biomass on char
flow rate and overall performance of BG, SOFC and GT system
is examined. Energy analysis is applied by thermodynamic

model. Regarding entire char conversion and acceptable system efficiency, the suggested operating conditions are
proposed.

2.

System description

A schematic of an integrated biomass gasification, SOFC and
GT system is shown in Fig. 1. Biomass enters a dryer and its
moisture content is reduced to a level acceptable by gasifier.
Air, oxygen and steam may be used as gasification agents. In
this work air enters a downdraft gasifier. The syngas produced
by gasification is cleaned up after entering a hot gas cleaning
unit according to the tolerance limits of SOFC. Then, the
cleaned syngas enters the SOFC, where electricity is produced.
The depleted fuel and air enter a combustor to burn. The high
temperature and pressure effluent from the combustor is
expanded through GT to generate mechanical power, which is
used to generate electrical power. The GT exhaust is used to
increase the temperature of air supplied by compressor to the

Fig. 1 e Integrated biomass gasifier, SOFCs and GT system.


b i o m a s s a n d b i o e n e r g y 7 5 ( 2 0 1 5 ) 3 5 e4 5

37

SOFC. Then the stream of burned gas supplies heat to a steam
generator, where feed water for user takes up the heat up to

its corresponding saturation temperature at pressure of
121.59 kPa. Finally, the stream gives heat to the dryer and goes
into the atmosphere.

In order to analyze the effects of air supply and moisture
content of biomass on process of gasification, moisture content (MC) of biomass and equivalence ratio (ER) are defined as

3.

Model description

ER ¼

3.1.

Dryer

To solve the problem, equilibrium reactions are required.
The two equilibrium reactions in the pyrolysis-oxidant zone
are

In order to analyze drying of wet biomass prior to gasification,
it is assumed that the initial moisture content of wet biomass
is 40%. After drying, wet biomass in which water mass fraction of 10%e30% enters a gasifier. The chemical equation the
dryer is shown as:
CHa Ob Np þ wtotal H2 OðlÞ ¼ CHa Ob Np þ wH2 OðlÞ þ wv H2 OðvÞ

(1)

The enthalpy of evaporation for water is 44.011 kJ molÀ1 at


25 C.

3.2.

Gasifier

The structure of a downdraft gasifier in this work is shown in
Fig. 2, the dimensions of the gasifier are similar to that from
Jayah et al. [19].
The gasifier is divided into two parts: pyrolysis-oxidation
zone where pyrolysis and oxidation reactions take place and
reduction zone, where the reduction reactions occur. The
output data from the exit of the pyrolysis -oxidation zone are
transferred as input data to entrance of the reduction zone.

3.2.1.

Model of pyrolysis-oxidation zone

The global reaction in the pyrolysis-oxidation zone can be
written as
CHa Ob Np þ wH2 O þ mðO2 þ 3:76N2 Þ
¼ x1 H2 þ x2 CO þ x3 CO2 þ x4 H2 O þ x5 CH4 þ x6 N2 þ x7 C

(2)

MC ¼

Masswater

18w
¼
Masswaterþbiomass ð12 þ a þ 16b þ 14pÞ þ 18w

Airactural
m
¼
Airstoichiometric 1 þ 0:25a À 0:5b

(3)

(4)

C þ 2H2 4CH4

(5)

CO þ H2 O4H2 þ CO2

(6)

The equilibrium constants for them are
K1 ¼


h 
i
x5 nT
¼ exp À G0T;CH4 À 2G0T;H2 =ðRm TÞ
2

x1 P0

(7)

K2 ¼

h 

i
x1 x3
¼ exp À G0T;H2 þ G0T;CO2 À G0T;CO À G0T;H2 O =ðRm TÞ
x2 x4

(8)

where nT is total mole of the syngas, P0 is total pressure.
The energy balance equation can be written as (assuming
no heat loss and work ¼ 0)
Hbiomass þ wHH2 O þ mHO2 þ 3:76mHN2
¼ x1 HH2 þ x2 HCO þ x3 HCO2 þ x4 HH2 O þ x5 HCH4 þ x6 HN2 þ x7 HC
(9)
The values of unknownsx1, x2, x3, x4, x5, x6, x7 and the reaction temperature T are determined by eight equations.
These equations are four atom balances, one fixed carbon
balance, two chemical equilibrium equations and one energy
balance equation. The values of the thermodynamic properties are adopted from Perry [20].
Once tow equilibrium constants are calculated at a
tentative temperature, thex1, x2, x3, x4, x5, x6 ,x7 are

Fig. 2 e Schematic diagram of a downdraft gasifier and reduction zone for calculation.

38

b i o m a s s a n d b i o e n e r g y 7 5 ( 2 0 1 5 ) 3 5 e4 5

determined by solving the equations using NewtoneRaphson
method. Then, the temperature is obtained by bisection
method. This temperature is taken as the initial temperature
for the next iteration until a specified convergence criterion is
obtained.

3.3.

Filter and scrubber

The output data from the exit of the pryo-oxidation zone is
transferred as input data to entrance of the reduction zone.
The control volumes of reduction zone for calculation are
shown in Fig. 2.
The reduction reactions considered in this zone are

The syngas consists impurities such as tar, sulphur and other
contaminant which may cause the degradation of SOFC. A gas
cleanup unit should be used to clean the syngas. There are two
options, hot and cold gas cleanup subsystems are supplied,
which can be found in Ref. [24].
In this study, a hot gas cleanup unit is chosen. After
entering filter and scrubber, the syngas are suitable to be used
in SOFC. To simplify the calculation, the mass balances of
syngas in filter and scrubber are ignored, the mass flow rate of

products is supposed to be constant.

R1 : C þ CO2 42CO

(10a)

n3 ¼ n4

R2 : C þ H2 O4CO þ H2

(10b)

R3 : C þ 2H2 4CH4

(10c)

3.2.2.

Model of reduction zone

(10d)

These four chemical reactions are considered to be
reversible. The specific reaction rates are expressed as kinetic
rate equations [21,22]. The kinetic rate parameters are obtained as reported by Wang and Kinoshita [23]. Thus the
volumetric reaction rate of each chemical reaction can be
written as




ÀER1
y2
yCO2 À CO
rR1 ¼ CRF AR1 exp
Rm T
KR1

(11a)




ÀER2
yH yCO
yH2 O À 2
rR2 ¼ CRF AR2 exp
Rm T
KR2

(11b)

rR3 ¼ CRF AR3

rR4 ¼ CRF AR4

After leaving the filter and scrubber, the temperature of the
syngas is decreased to the level of that at anode inlet. Heat loss
of gas cleanup unit may be written as
Q¼

R4 : CH4 þ H2 O43H2 þ CO




ÀER3
yCH4
y2H2 À
exp
Rm T
KR3
!


yCO y3H2
ÀER4
yH2 O yCH4 À
exp
Rm T
KR4

(14)

6
X

n3;i $H3;i À

i¼1

3.4.

6
X

n4;i $H4;i

(15)

i¼1

Solid oxide fuel cell

In general, the ideal reversible potential of H2eO2 SOFC can be
determined by the Nernst equation
 1=2
ÀDG0 RT pH2 $ pO2
ln
þ
(16)
E0 ¼
2F
2F
pH2 O
Nernst potential is reduced to the terminal voltage by the
sum of the local voltage polarizations. The three polarizations
are ohmic, activation and concentration polarization. Therefore the cell terminal voltage is given by
V ¼ E0 À hact;a À hact;c À hohm À hcon

(11c)

(11d)

(17)

The activation polarizations of anode and cathode have
been given in literature [25]. Ohmic polarization is expressed
by Ohm's law as shown in Ref. [26].
In the SOFC, the overall electrochemical is as follows,
which is significantly exothermic.

The mass balance for the species i across the control volume k can be expressed as

1
H2 þ O2 4H2 O
2

nki ¼ nikÀ1 þ Rki DVk

For a BG-SOFC system, usual high operating temperature of
SOFC allows sustaining the reforming and shifting reactions
as follows to produce hydrogen.

(12)

where nki is molar flow rate (mol sÀ1),Rki is the net rate of production of species i (mol mÀ3 sÀ1): for exampleRkH2 ¼ rR2 À 2rR3 þ 3rR4 ,RkC ¼ ÀrR1 À rR2 À rR3 , etc., DVk is
volume of the kth control volume (m3).
The energy balance on the element can be expressed as
6
X

i¼1

6
À
Á X
À
Á
nikÀ1 HkÀ1
þ n7kÀ1 Cp;C TkÀ1 À T0 ¼
nki Hki þ nk7 Cp;C Tk À T0
i

(18)

CO þ H2 O4CO2 þ H2

(19)

CH4 þ H2 O4CO þ 3H2

(20)

The electric power produced in SOFC is given by
WSOFC ¼ IV

i¼1

(13)
Once the equilibrium constants KR1ÀKR4 are calculated at a
tentative temperature, Rki is determined and nki is given by Eq.

(12). Then, the gasification temperature of the kth control
volume of the reduction zone is obtained by Eq. (13) using
bisection method. This temperature is taken as the initial
temperature for the next iteration until a specified criterion is
satisfied.

(21)

The equation for the energy balance of SOFC is
X
i

Hin
i þ

X
k

Rk ðÀDHk Þ ¼ WSOFC þ

X

Hout
i

(22)

i

The energy balance includes the electrical power WSOFC and

the enthalpy changes of the chemical and electrochemical reactions, and gives the evaluation of the average temperature of
the stack. The detailed description of the electrochemical
simulation of SOFC could be found in Refs. [27,28].


b i o m a s s a n d b i o e n e r g y 7 5 ( 2 0 1 5 ) 3 5 e4 5

3.5.

Combustor

3.7.

The depleted fuel and air from SOFC enter a combustor to
burn for heat recovery. Enough oxygen is supplied so that all
unreacted fuel from SOFC can be consumed. That is to say,
complete combustion occurs in the combustor. The energy
balance about the combustor is expressed as
0
0
1
1
ZTin
ZTout
7
4
X
X
in @
0

out @
0
A
DHf þ
ni DHf þ
Cp dT ¼
ni
Cp dTA
(23)
i¼1

298

i¼1

Micro gas turbine and compressor

Model of gas turbine and compressor are well described in the
literature [10]. To simply the study, it is assumed that the gas
turbine and compressor work at their respective designed
condition under steady-state operation. A set of operating
parameters and the assumed efficiencies are given in Table 1.
Once the pressure ratio is given, the outlet temperature of the
compressor and gas turbine is given as:
TCOM;out
TGT;in
kÀ1
¼
¼pk
TCOM;in

TGT;out

(24)

Then the compressor work and gas turbine output can be
obtained, respectively
WCOM ¼

1
½HðTCOM;out Þ À HðTCOM;out ފ
hCOM;s

(25)

Ã
 Á
WGT ¼ hGT;s H TGT;in À HðTGT;out Þ

(26)

where, hs is isentropic efficiency given in Table 1.

Table 1 e Operating conditions.
Environmental
Ambient temperature
Ambient pressure
Biomass data
Type of biomass
Ultimate analysis (wt%)

Moisture content in biomass
Mass flow rate of dry biomass
Gasifier
Gasifier operating pressure
Moisture content of biomass
entering gasifier (State 2)
Equivalence ratio
Molar fraction of air
SOFC
SOFC operating temperature
Anode inlet temperature
Fuel utilization Uf
DC/AC inverter efficiency
Peripheral equipment
Isentropic efficiency of compressor
Pressure ratio of compressor
Isentropic efficiency of GT
Outlet temperature of GT (State 11)
Pressure ratio of water pump
Exhaust temperature (State 14)

Energy efficiencies

The performance of BG，SOFC and GT power systems can be
evaluated by energy efficiencies. Energy efficiency is defined as
the ratio of useful energy products to total energy inputs [29].
Net electrical power output of the system is expressed as:
Wnet ¼ WSOFC þ WGT À WCompressorÀ1 À WCompressorÀ2 À WPump

(27)

The heating production for user in Fig. 1 is given as:

298

Then the adiabatic combustion temperature can be determined from Eq. (23).

3.6.

39

25  C
101.325 kPa
Wood
50% C, 6% H, 44% O
40%
10e30 kg hÀ1
253.313 kPa
10%e30%
0.39e0.5
21%O2,79%N2
800  C
750  C
0.85
95%
0.75
2.5
0.85
790  C
1.2

130  C

Q ¼ n15 $ðH17 À H15 Þ

(28)

Therefore, the electrical efficiency, combined heat and
power efficiency can be calculated by Equations (29) and (30),
respectively.

hel ¼

Wnet
nbiomass $LHVbiomass

hCHP ¼

4.

Wnet þ Q
nbiomass $LHVbiomass

(29)

(30)

Results and discussion

The output data from the exit of gasifier are transferred as
input data to entrance of the SOFC. The key parameter in SOFC

computation is the air utilization ratio which is dependent on
various operating and design data. The electrochemical model
determines terminal voltage and electric power. The energy
balance Eq. (22) accepts these results from electrochemical
model and calculates a new molar flow rate of air at the
cathode inlet. The air utilization ratio is applied to the electrochemical model for the next calculation of cell terminal
voltage and power until the convergence is obtained.
For the whole system model, since the calculation of heat
exchanger need the heating fluid parameters such as the gas
temperature at the combustor exit, which are not known at
the beginning of the simulation, a set of initial parameters has
to be assumed in order to run the system model until
convergence is met eventually. A set of operating parameters
and the assumed efficiencies of the system components are
given in Table 1. The power system is simulated using Matlab
7.0.
The present model has been validated against the experimental results of Jayah et al. [19]. Comparison of predicted and
measured gas composition at gasifer exit is shown in Fig. 3.
Comparison of the temperature distribution with the experimental result is shown in Fig. 4. The species concentrations at
the gasifier exit are obtained from the data of the last control
volume of the reduction zone.
This work did not take into account the heat loss in the
gasifier, the molar fractions of CO, H2 and CH4 contents are
slightly higher than the real values. At the same time, the
value of N2 is slightly less than that of experiment. However,
the good agreement between the model prediction and the
experiment shows the present model is reliable.
A parametric analysis is carried out to study the effects of
the process parameters (MC, ER and mass flow rate) on the
overall performance of the power system.

40

b i o m a s s a n d b i o e n e r g y 7 5 ( 2 0 1 5 ) 3 5 e4 5

Fig. 3 e Comparison of predicted and measured gas
composition at gasifier exit.

4.1.

Fig. 5 e Variation of char flow rate along the height of
reduction zone for different moisture contents.

Effect of MC

Moisture content is one of the important parameters since
most of the biomass contains high percentage of moisture. In
this study the original MC is assumed to be 40%. After drying,
MC varies between 10% and 30% before entering gasifier. As
effect of MC on the performance of power system is analyzed,
only the studied parameter is changed, ER, operating pressure
of gasifier and mass flow rate of dry biomass are constant as
_ ¼ 20 kg hÀ1, other input data
ER ¼ 0.42, P ¼ 253.313 kPa andm
are assumed as in Table 1.
Figs. 5 and 6 show the effect of MC on char flow rate and
temperature along the height reduction zone. It is observed
that the temperature decreases from the entrance to the exit
of reduction zone and remains lower with higher MC. As MC is

higher, much heat generated in gasifier is used to evaporate
the moisture and superheat the vapour, which resulting in the
decreasing of gasifier temperature. Accordingly, the char flow
rate is lower with lower MC, because the lower temperature is
unfavourable for char conversion. It is seen all the char is
consumed in the reduction zone as MC less than 0.2, while
24% of char is left at the exit of gasifier as MC equal to 0.3.
Therefore, the biomass should be dried to the level of 10e20%
for moisture before gasification.
Effect of MC on syngas composition is shown in Table 2.
Power input and output, combustor temperature, net power
and heat output are also shown in Table 2.

Fig. 4 e Comparison of predicted and measured
temperatures along the height of the reduction zone.

It can be seen from Table 2 that the molar fraction of H2 and
CO decrease while the content of H2O and CO2 increase with
the increasing of MC.
As seen from the Table 2, the molar fraction of the CH4 is
far less than other gases at the gasifier exit, most of the carbon
in the biomass is converted into CO. At the anode exit of SOFC
(State 5), the concentration of CO decrease according to the
mildly exothermic water-gas shift reaction in SOFC, the concentration of H2 decreases due to the electrochemical reaction, the concentration of H2O increases accordingly.
The output power of SOFC decreases due to the decreasing
of molar ratio of H2 to H2O at the anode inlet as MC increasing
which leads to the lower terminal voltage of SOFC. The higher
the molar fraction of H2O at the exit of SOFC anode, the lower
the temperature of the combustor (State 10). The temperature
falls by 17 K as shown in Table 2 as MC varying from 0.1 to 0.2.

It results in the decline of the output of GT. Both of the
decreasing of output power of SOFC and GT determine the
reduction of the system net power. On the other hand, with
higher MC the heat provided to dryer to evaporate the moisture is less, therefore, more heat is left to steam generator.
The overall performance of the BG-SOFC-GT system is also
shown in Table 2. The electrical efficiency decreases by 6% as
MC increasing from 0.1 to 0.2. The electrical efficiency is above
40% as long as MC less than 0.2. The heat efficiency increases

Fig. 6 e Variation of temperature along the height of
reduction zone for different moisture contents.


41

b i o m a s s a n d b i o e n e r g y 7 5 ( 2 0 1 5 ) 3 5 e4 5

Table 2 e Effect of moisture contents on gas composition
and performance of the power system.
MC ¼ 0.1

H2 (%)
CO (%)
CH4 (%)
CO2 (%)
H2O (%)
N2 (%)
WC1 (W)
WC2 (W)
WSOFC (W)

T(K)
(State 10)
WGT (W)
Wnet (W)
Heat (W)
hel (%)
hCHP (%)

MC ¼ 0.15

MC ¼ 0.2

State
(3)

State
(5)

State
(3)

State
(5)

State
(3)

State
(5)

18.63
17.96
0.003
11.48
5.02
46.91
1724.5
21,251
35,098
1204

4.58
6.06
0.01
23.38
19.07
46.90

17.21
17.95
0.003
10.55
8.30
45.98
1724.5
21,866
34,431
1195

4.48

5.33
0.01
23.17
21.03
45.98

15.88
17.15
0.003
10.09
11.73
45.14
1724.5
21,790
33,067
1187

4.28
4.50
0
22.74
23.34
45.14

34,066
46,189
20,799
46.54
67.49

32,842
43,682
22,456
44.01
66.63

30,739
40,291
23,953
40.59
64.73

from 21% to 24%.As a result, combined heat and power
efficiency decreases from 67% to 64% as MC increasing from
0.1 to 0.2.

4.2.

Effect of ER

Effect of equivalence ratio on char flow rate and temperature
along the reduction zone are shown in Figs. 7 and 8, respectively. MC and pressure of gasifier are constant as MC ¼ 0.2,
_ ¼ 20 kg hÀ1, other input data are
P ¼ 253.313 kPa andm
assumed as in Table 1.
It is seen that conversion of char is more remarkable with
higher ER, owing to higher reaction temperature, which
determining the extent of carbon conversion. All the char is
consumed in reduction zone on the condition of ER more than
0.42.

Gas composition, power input and output, combustor
temperature, net power and heat output for different ER is
shown in Table 3. It shows that the molar fraction of H2, CO2
and N2 increase slightly with higher ER, whereas a significant
decrease of the molar fraction of H2O occurs. Therefore, the

Fig. 8 e Variation of temperature along the height of
reduction zone for different equivalence ratios.

higher molar ratio of H2 to H2O results in the increasing of
output power from SOFC. The increasing of output power of
GT is due to the higher combustor temperature with higher
ER. Although more power given to compressors are required,
the overall useful output power from SOFC and GT overweighs
the input power for compressors.
Effect of ER and MC on char conversion is shown in Fig. 9.
The entire conversion of char is gained if ER and MC is selected
above the line.
Effect of ER and MC on overall performance of the BGSOFC-GT system are shown in Figs. 10 and 11. Both of electrical and CHP efficiencies increase with higher ER. For
example, the electrical efficiency increase from 41% to 45%,
the CHP efficiency from 65% to 71% as ER changing from 0.42
to 0.46 when moisture content and mass flow rate of dry
_ ¼ 20 kg hÀ1.
biomass are constants as MC ¼ 0.2 and m

4.3.

Effect of mass flow rate of dry biomass

The char flow rate along the height of the reduction zone for

different mass flow rates of dry biomass is shown in Fig. 12. In
Fig. 12, ER and MC are constant as ER ¼ 0.42 and MC ¼ 0.2,

Table 3 e Effect of ER on syngas composition and
performance of the power system.

Fig. 7 e Variation of char flow rate along the height of
reduction zone for different equivalence ratios.

H2 (%)
CO (%)
CH4 (%)
CO2 (%)
H2O (%)
N2 (%)
WC1 (W)
WC2 (W)
WSOFC (W)
T(K) (State 10)
WGT (W)
Wnet (W)
Heat (W)
hel (%)
hCHP (%)

ER ¼ 0.4

ER ¼ 0.42

ER ¼ 0.44

15.57
17.24
0.004
9.67
13.01
44.50
1642.4
21,262
31,796
1184
29,316
38,208
23,054
35.49
61.72

15.88
17.15
0.003
10.09
11.73
45.14
1724.5
21,790
33,067
1187
30,739
40,291
23,953

40.59
64.73

16.37
16.87
0.002
10.67
10.35
45.74
1806.6
22,215
34,359
1190
32,215
42,552
24,762
42.87
67.82


42

b i o m a s s a n d b i o e n e r g y 7 5 ( 2 0 1 5 ) 3 5 e4 5

Fig. 9 e Effect of ER and MC on char conversion.

Fig. 10 e Effect of ER and MC on electrical efficiency.

other input data are assumed as in Table 1. Effect of mass flow
rate on syngas composition and electrical and CHP efficiencies

is shown in Table 4.
It is shown that char conversion is more active with
smaller mass flow rate. All the char get consumed completely
in the range of less than 20 kg hÀ1. As mass flow rate
decreasing, molar fraction of H2 and CO increase, however all
these species don't show significant variation as mass flow
rate changing from 15 kg hÀ1 to 25 kg hÀ1. Although the total

Fig. 12 e Variation of char flow rate different mass flow rate
of dry biomass.

work from SOFC and GT and heat are enhanced as mass flow
rate increasing, the mass flow rate increase significantly, the
electrical and CHP efficiencies are reduced ultimately. The
electrical efficiency is above 41% in the range of 10 kg hÀ1 to
20 kg hÀ1.
Effect of ER and mass flow rate on char conversion is
shown in Fig. 13. The entire conversion of char is gained if ER
and mass flow rate is selected above the line.
Effect of ER and mass flow rate of dry biomass on overall
performance of the BGeSOFCeGT system are shown in Figs.
14 and 15. Both of electrical and CHP efficiencies increase
with smaller mass flow rate. For example, the electrical efficiency increase from 41% to 46%, the CHP efficiency from 65%
to 71% as mass flow rate changing from 20 kg hÀ1 to 10 kg hÀ1
when ER and MC are constants as MC ¼ 0.2 and ER ¼ 0.42.
Many variables affect the overall system's electric and CHP
efficiencies. The total plant performance can be compared to
the results of other literature. Colpan et al. [12,30] studied the
effect of gasification agent (air, enriched oxygen and steam)
on the performance of an integrated SOFC and BG system.

The results show that the electrical efficiency of the system is
25% with superheated steam and pre-heated air as gasification agent and the highest electrical efficiency of 41.8% could
be gained using steam as the gasification agent. The electrical

Table 4 e Effect of mass flow rate on syngas composition
and performance of the power system.

Fig. 11 e Effect of ER and MC on CHP efficiency.

H2 (%)
CO (%)
CH4 (%)
CO2 (%)
H2O (%)
N2 (%)
WC1 (W)
WC2 (W)
WSOFC (W)
WGT (W)
Wnet (W)
hel (%)
hCHP (%)

_ ¼ 15kg hÀ1
m

_ ¼ 20kg hÀ1
m

_ ¼ 25kg hÀ1

m

16.45
17.90
0.004
9.92
10.94
44.79
1293.4
17045
26,004
24,297
31,962
42.94
67.59

15.88
17.15
0.003
10.09
11.73
45.14
1724.5
21790
33,067
30,739
40,291
40.59
64.73

15.43
16.57
0.003
10.22
12.35
45.42
2155.6
26,316
39,805
36,923
48,256
38.89
62.56


b i o m a s s a n d b i o e n e r g y 7 5 ( 2 0 1 5 ) 3 5 e4 5

Fig. 13 e Effect of ER and mass flow rate on char
conversion.

and 50.8% (LHV). A hybrid plant consisting BG, SOFC and
organic Rankine cycle has been reported in Ref. [11] with
electrical efficiency of 54%, which close to the highest electrical efficiency of 56% in our study. However, the electrical
efficiency of BG-SOFC-MGT system reached a level of 58% has
been shown by some researchers [13], meanwhile, the CHP
efficiency of 87.5% in their study is more than the highest
CHP efficiency of 85% in this study. The improvement comes
mainly from the optimization efforts in heat exchanger
network and decreasing of the temperature of the exhaust
gas leaving the hybrid plant in their study. The temperature

of the exhaust gas in their study is reduced from 120  C to
90  C, while the exhaust gas is set as 130  C in this study
avoiding stack corrosion.
It should be noticed that all of these authors mentioned
above did not take into account any char conversion in the
reduction zone of gasifier as mentioned in the introduction.

5.

Fig. 14 e Effect of ER and mass flow rate on electrical
efficiency.

efficiencies are below 45% due to the absence of gas turbine
in their systems. In this study, the electrical efficiency is always above 46% in the range of MC < 0.2, ER > 0.46 and mass
flow rate less than 20 kg hÀ1. Sucipta et al. [6] reported a
combined SOFC and MGT system fed with syngas from
biomass gasifier with electrical efficiencies between 46.4%

43

Conclusions

An integrated BG, SOFC and GT system is investigated by
thermodynamic model. The pyrolysis-oxidant zone of the
gasifier is modeled based on chemical equilibrium, reaction
temperature is determined considering thermal equilibrium.
Meanwhile, kinetic reaction model has been adopted for onedimensional simulation of the reduction zone, the temperature and char flow rate along the height of reduction zone
have been predicted. Effects of process parameters (MC, ER
and mass flow rate) on performance of the whole system have
been studied.

It is found that char in the biomass tends to be converted
with decreasing of MC and increasing of ER due to higher
temperature in reduction zone. The entire conversion of char
in biomass could be expected on the condition of MC < 0.2,
ER > 0.42 and m_ < 20 KghÀ1 .
The electric and CHP efficiencies of the power system increase with decreasing of MC and the increasing of ER, the
electrical efficiency of this system could reach a level of
approximately 56%.
Although the lower mass flow rate of biomass is favorable
for char conversion and improvement of system efficiencies,
it means that a larger gasifier has to be adopted and the capital
cost of the equipment will increase.
Regarding the entire conversion of char in gasifier and
acceptable electrical efficiency above 45%, the operating condition in this study is suggested to be in the range of MC less
than 0.2, ER more than 0.46 and mass flow rate of biomass less
than 20 kg hÀ1.
The future work about this study will include a more
comprehensive multi-dimensional gasifier model considering
tar production and optimization study of the power system
configuration.

Acknowledgment
Fig. 15 e Effect of ER and mass flow rate on CHP efficiency.

The authors are grateful for the support of the Centre College
Primary Scientific Research Item Funds (HEUCF130311,


44

b i o m a s s a n d b i o e n e r g y 7 5 ( 2 0 1 5 ) 3 5 e4 5

HEUCF140311, HEUCFZ1103), National Natural Science fund of
China (51009039, 51179037) and Harbin Science and Technology Bureau (RC2013XK008002, 2013RFXXJ050).

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Nomenclature
AR: pre-exponential factor, mol mÀ3 sÀ1
Cp: specific heat at constant pressure, J molÀ1 KÀ1
CRF: char reactivity factor
ER: activation energy, J molÀ1
ER: equivalence ratio
E0: reversible cell potential, V
F: Faraday constant, 96,485C molÀ1
G: Gibbs function, J molÀ1
DG: change in Gibbs free energy, J molÀ1
H:: enthalpy, J molÀ1
DH:: enthalpy change of reaction, J molÀ1
I: current, A
k: specific heat ratio
K: equilibrium constant

LHV: lower heating value, J molÀ1
_ mass flow rate of biomass, kg hÀ1
m:
MC: moisture content
n: Molar flow rate, mol sÀ1
P: Pressure, Pa
Q: Heat, W
r: volumetric reaction rate, mol mÀ3 sÀ1
R: universal gas constant, 8.314 J molÀ1 KÀ1
Ri: net rate of production of species i, mol mÀ3 sÀ1
T: temperature, K
Uf: fuel utilization
DVk: volume of the kth control volume, m3
V: terminal voltage of fuel cell, V
y: molar fraction
W: electrical power, W


b i o m a s s a n d b i o e n e r g y 7 5 ( 2 0 1 5 ) 3 5 e4 5

Greek Letters
h: polarization, V
h: efficiency, %
p: pressure ratio
Subscripts
a: anode

act: activation polarization
c: cathode
com: compressor

con: concentration polarization
ohm: ohm polarization
GT: gas turbine
COM: compressor
CHP: combined heat and power

45