Effect of operating parameters on performance ofan integrated biomass gasifier, solid oxide fuel cells and micro gas turbine system Junxi Jiaa,*, Abuliti Abudulab, Liming Weic, Baozhi Su
Trang 1Effect of operating parameters on performance of
an integrated biomass gasifier, solid oxide fuel cells
and micro gas turbine system
Junxi Jiaa,*, Abuliti Abudulab, Liming Weic, Baozhi Suna, Yue Shia
aCollege of Power and Energy Engineering, Harbin Engineering University, Harbin 150001, China
bNorth Japan Research Institute for Sustainable Energy, Hirosaki University, Aomori 030-0813, Japan
cSchool of Electric and Electronic Information Engineering, Jilin Jianzhu University, Changchun 130118, China
a r t i c l e i n f o
Article history:
Received 13 October 2014
Received in revised form
5 February 2015
Accepted 6 February 2015
Available online
Keywords:
Biomass gasification
Solid oxide fuel cell
Chemical equilibrium
Kinetics model
Combined heat and power
a b s t r a c t
An integrated power system of biomass gasification with solid oxide fuel cells (SOFC) and micro gas turbine has been investigated by thermodynamic model A zero-dimensional electrochemical model of SOFC and one-dimensional chemical kinetics model of down-draft biomass gasifier have been developed to analyze overall performance of the power system Effects of various parameters such as moisture content in biomass, equivalence ratio and mass flow rate of dry biomass on the overall performance of system have been studied by energy analysis
It is found that char in the biomass tends to be converted with decreasing of moisture content and increasing of equivalence ratio due to higher temperature in reduction zone of gasifier Electric and combined heat and power efficiencies of the power system increase with decreasing of moisture content and increasing of equivalence ratio, the electrical efficiency of this system could reach a level of approximately 56%.Regarding entire 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 h1
© 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
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 environ-mental 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 pro-duction since it combines the merits of renewable energy sources and hydrogen energy systems
* Corresponding author
E-mail address:jiajunxi99@sohu.com(J Jia)
Available online at www.sciencedirect.com
ScienceDirect
http://www.elsevier.com/locate/biombioe
http://dx.doi.org/10.1016/j.biombioe.2015.02.004
0961-9534/© 2015 Elsevier Ltd All rights reserved
Trang 2Thermodynamic 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
ox-ygen 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 H2utilization of SOFC and moisture content in
biomass have been analyzed in details In their work, a
zero-dimensional chemical equilibrium model was used in
gasifier The authors did not take into account any char
con-version in the reduction zone of gasifier
Most of gasification models adopted to analyze the
per-formance 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 tem-perature 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 pre-sented in order to overcome the limitations of the equilibrium model The gas composition, reaction temperature, and un-reacted 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 sys-tem efficiency, the suggested operating conditions are proposed
2 System description
A schematic of an integrated biomass gasification, SOFC and
GT system is shown inFig 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 1e Integrated biomass gasifier, SOFCs and GT system
Trang 3SOFC 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
3 Model description
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
frac-tion of 10%e30% enters a gasifier The chemical equafrac-tion the
dryer is shown as:
CHaObNpþ wtotalH2OðlÞ¼ CHaObNpþ wH2OðlÞþ wvH2OðvÞ (1)
The enthalpy of evaporation for water is 44.011 kJ mol1at
25C
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
CHaObNpþ wH2Oþ mðO2þ 3:76N2Þ
¼ x1H2þ x2COþ x3CO2þ x4H2Oþ x5CH4þ x6N2þ x7C (2)
In order to analyze the effects of air supply and moisture content of biomass on process of gasification, moisture con-tent (MC) of biomass and equivalence ratio (ER) are defined as
MC¼ Masswater
Masswaterþbiomass¼ð12 þ a þ 16b þ 14pÞ þ 18w18w (3)
ER¼ Airactural
To solve the problem, equilibrium reactions are required The two equilibrium reactions in the pyrolysis-oxidant zone are
The equilibrium constants for them are
K1¼x5nT
x2P0¼ exphG0
T;CH 4 2G0
T;H 2
K2¼x1x3
x2x4¼ exphG0
T;H 2þ G0 T;CO 2 G0 T;CO G0 T;H 2 O
=ðRmTÞi (8) where nTis total mole of the syngas, P0is total pressure The energy balance equation can be written as (assuming
no heat loss and work¼ 0)
Hbiomassþ wHH 2 Oþ mHO 2þ 3:76mHN 2
¼ x1HH 2þ x2HCOþ x3HCO 2þ x4HH 2 Oþ x5HCH 4þ x6HN 2þ x7HC
(9) The values of unknownsx1, x2, x3, x4, x5, x6, x7and the re-action 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 proper-ties are adopted from Perry[20]
Once tow equilibrium constants are calculated at a tentative temperature, thex1, x2, x3, x4, x5, x6 ,x7 are
Fig 2e Schematic diagram of a downdraft gasifier and reduction zone for calculation
Trang 4determined 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
3.2.2 Model of reduction zone
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 inFig 2
The reduction reactions considered in this zone are
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
ob-tained as reported by Wang and Kinoshita [23] Thus the
volumetric reaction rate of each chemical reaction can be
written as
rR1¼ CRFAR1exp
ER1
RmT
yCO 2y2CO
KR1
(11a)
rR2¼ CRFAR2exp
ER2
RmT
yH 2 OyH 2yCO
KR2
(11b)
rR3¼ CRFAR3exp
ER3
RmT
y2
H 2yCH 4
KR3
(11c)
rR4¼ CRFAR4exp
ER4
RmT
yH 2 OyCH 4yCOy
3
H 2
KR4
!
(11d) The mass balance for the species i across the control
vol-ume k can be expressed as
nk
i ¼ nk1
i þ Rk
where nk
i is molar flow rate (mol s1),Rk
i is the net rate of
exam-pleRk
H 2¼ rR2 2rR3þ 3rR4,Rk
C¼ rR1 rR2 rR3, etc., DVk is volume of the kth control volume (m3)
The energy balance on the element can be expressed as
X6
i¼1
nk1i Hk1i þ nk1
7 Cp;C
Tk1 T0
¼X6
i¼1
nk
iHk
i þ nkCp;C
Tk T0 (13) Once the equilibrium constants KR1KR4are calculated at a
tentative temperature, Rk
i is determined and nk
i 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
3.3 Filter and scrubber 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
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
Q¼X6
i¼1
n3;i$H3;iX6
i¼1
3.4 Solid oxide fuel cell
In general, the ideal reversible potential of H2eO2SOFC can be determined by the Nernst equation
E0¼DG0
2Fln
pH 2$pO 2
1=2
pH 2 O
(16) 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 There-fore the cell terminal voltage is given by
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
H2þ1
For a BG-SOFC system, usual high operating temperature of SOFC allows sustaining the reforming and shifting reactions
as follows to produce hydrogen
The electric power produced in SOFC is given by
The equation for the energy balance of SOFC is X
i
Hin
k
RkðDHkÞ ¼ WSOFCþX
i
Hout
The energy balance includes the electrical power WSOFCand the enthalpy changes of the chemical and electrochemical re-actions, 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]
Trang 53.5 Combustor
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
X7
i¼1
nin
i
0
@DH0
fþ Z
T in
298
CpdT
1
A ¼X4
i¼1
nout i
0
@DH0
fþ Z
T out
298
CpdT
1
Then the adiabatic combustion temperature can be
deter-mined from Eq.(23)
3.6 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 inTable 1
Once the pressure ratio is given, the outlet temperature of the
compressor and gas turbine is given as:
TCOM;out
TCOM;in ¼ TGT;in
TGT;out¼ pk1
Then the compressor work and gas turbine output can be
obtained, respectively
WGT¼ hGT;s
H
TGT;in
where,hsis isentropic efficiency given inTable 1
3.7 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:
Wnet¼ WSOFCþ WGT WCompressor1 WCompressor2 WPump (27) The heating production for user inFig 1is given as:
Therefore, the electrical efficiency, combined heat and power efficiency can be calculated by Equations(29) and (30), respectively
nbiomass$LHVbiomass
(29)
hCHP¼ Wnetþ Q
nbiomass$LHVbiomass
(30)
4 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 elec-trochemical 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 inTable 1 The power system is simulated using Matlab 7.0
The present model has been validated against the experi-mental results of Jayah et al.[19] Comparison of predicted and measured gas composition at gasifer exit is shown inFig 3 Comparison of the temperature distribution with the experi-mental result is shown inFig 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, H2and CH4contents are slightly higher than the real values At the same time, the value of N2is 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
Environmental
Ambient temperature 25C
Biomass data
Ultimate analysis (wt%) 50% C, 6% H, 44% O
Moisture content in biomass 40%
Mass flow rate of dry biomass 10e30 kg h1
Gasifier
Gasifier operating pressure 253.313 kPa
Moisture content of biomass
entering gasifier (State 2)
10%e30%
Molar fraction of air 21%O2,79%N2
SOFC
SOFC operating temperature 800C
Anode inlet temperature 750C
DC/AC inverter efficiency 95%
Peripheral equipment
Isentropic efficiency of compressor 0.75
Pressure ratio of compressor 2.5
Isentropic efficiency of GT 0.85
Outlet temperature of GT (State 11) 790C
Pressure ratio of water pump 1.2
Exhaust temperature (State 14) 130C
Trang 64.1 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
ER¼ 0.42, P ¼ 253.313 kPa and _m¼ 20 kg h1, other input data
are assumed as inTable 1
Figs 5 and 6show 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 inTable 2
Power input and output, combustor temperature, net power
and heat output are also shown inTable 2
It can be seen fromTable 2that the molar fraction of H2and
CO decrease while the content of H2O and CO2increase with the increasing of MC
As seen from theTable 2, the molar fraction of the CH4is 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 con-centration of H2decreases due to the electrochemical reac-tion, the concentration of H2O increases accordingly The output power of SOFC decreases due to the decreasing
of molar ratio of H2to 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 inTable 2as 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 mois-ture is less, therefore, more heat is left to steam generator The overall performance of the BG-SOFC-GT system is also shown inTable 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 3e Comparison of predicted and measured gas
composition at gasifier exit
Fig 4e Comparison of predicted and measured
temperatures along the height of the reduction zone
Fig 5e Variation of char flow rate along the height of reduction zone for different moisture contents
Fig 6e Variation of temperature along the height of reduction zone for different moisture contents
Trang 7from 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 inFigs 7 and 8,
respec-tively MC and pressure of gasifier are constant as MC¼ 0.2,
P ¼ 253.313 kPa and _m ¼ 20 kg h1, other input data are
assumed as inTable 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 inTable 3 It shows that the molar fraction of H2, CO2
and N2increase slightly with higher ER, whereas a significant
decrease of the molar fraction of H2O occurs Therefore, the
higher molar ratio of H2to 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 inFig 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 BG-SOFC-GT system are shown inFigs 10 and 11 Both of elec-trical 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 biomass are constants as MC¼ 0.2 and _m¼ 20 kg h1
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 inFig 12 In
Fig 12, ER and MC are constant as ER ¼ 0.42 and MC ¼ 0.2,
and performance of the power system
State
(3)
State (5)
State (3)
State (5)
State (3)
State (5)
H2(%) 18.63 4.58 17.21 4.48 15.88 4.28
CO (%) 17.96 6.06 17.95 5.33 17.15 4.50
CH4(%) 0.003 0.01 0.003 0.01 0.003 0
CO2(%) 11.48 23.38 10.55 23.17 10.09 22.74
H2O (%) 5.02 19.07 8.30 21.03 11.73 23.34
N2(%) 46.91 46.90 45.98 45.98 45.14 45.14
WC1(W) 1724.5 1724.5 1724.5
WC2(W) 21,251 21,866 21,790
WSOFC(W) 35,098 34,431 33,067
T(K)
(State 10)
WGT(W) 34,066 32,842 30,739
Wnet(W) 46,189 43,682 40,291
Heat (W) 20,799 22,456 23,953
Fig 7e Variation of char flow rate along the height of
reduction zone for different equivalence ratios
Fig 8e Variation of temperature along the height of reduction zone for different equivalence ratios
performance of the power system
Trang 8other input data are assumed as inTable 1 Effect of mass flow
rate on syngas composition and electrical and CHP efficiencies
is shown inTable 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 h1 As mass flow rate
decreasing, molar fraction of H2and CO increase, however all
these species don't show significant variation as mass flow
rate changing from 15 kg h1to 25 kg h1 Although the total
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 h1to
20 kg h1 Effect of ER and mass flow rate on char conversion is shown inFig 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 inFigs
14 and 15 Both of electrical and CHP efficiencies increase with smaller mass flow rate For example, the electrical effi-ciency increase from 41% to 46%, the CHP effieffi-ciency from 65%
to 71% as mass flow rate changing from 20 kg h1to 10 kg h1 when ER and MC are constants as MC¼ 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 gasifica-tion agent and the highest electrical efficiency of 41.8% could
be gained using steam as the gasification agent The electrical
Fig 9e Effect of ER and MC on char conversion
Fig 10e Effect of ER and MC on electrical efficiency
Fig 11e Effect of ER and MC on CHP efficiency
Fig 12e Variation of char flow rate different mass flow rate
of dry biomass
and performance of the power system
_
m¼ 15kg h1 m_ ¼ 20kg h1 m_ ¼ 25kg h1
WSOFC(W) 26,004 33,067 39,805
Wnet(W) 31,962 40,291 48,256
Trang 9efficiencies are below 45% due to the absence of gas turbine
in their systems In this study, the electrical efficiency is
al-ways above 46% in the range of MC< 0.2, ER > 0.46 and mass
flow rate less than 20 kg h1 Sucipta et al [6] reported a
combined SOFC and MGT system fed with syngas from
biomass gasifier with electrical efficiencies between 46.4%
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 elec-trical efficiency of 56% in our study However, the elecelec-trical 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 120C 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
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 one-dimensional simulation of the reduction zone, the tempera-ture 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 Kgh1 The electric and CHP efficiencies of the power system in-crease 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 con-dition 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 h1
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
The authors are grateful for the support of the Centre College Primary Scientific Research Item Funds (HEUCF130311,
Fig 13e Effect of ER and mass flow rate on char
conversion
Fig 14e Effect of ER and mass flow rate on electrical
efficiency
Fig 15e Effect of ER and mass flow rate on CHP efficiency
Trang 10HEUCF140311, HEUCFZ1103), National Natural Science fund of
China (51009039, 51179037) and Harbin Science and
Technol-ogy Bureau (RC2013XK008002, 2013RFXXJ050)
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Nomenclature
AR: pre-exponential factor, mol m3s1
Cp: specific heat at constant pressure, J mol1K1
CRF: char reactivity factor
ER: activation energy, J mol1 ER: equivalence ratio
E0: reversible cell potential, V F: Faraday constant, 96,485C mol1 G: Gibbs function, J mol1
DG: change in Gibbs free energy, J mol1
H:: enthalpy, J mol1 DH:: enthalpy change of reaction, J mol1
I: current, A k: specific heat ratio K: equilibrium constant LHV: lower heating value, J mol1 _
m: mass flow rate of biomass, kg h1 MC: moisture content
n: Molar flow rate, mol s1 P: Pressure, Pa
Q: Heat, W r: volumetric reaction rate, mol m3s1 R: universal gas constant, 8.314 J mol1K1
Ri: net rate of production of species i, mol m3s1 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