Biomass fueling of a SOFC by integrated gasifier: Study of theGennaro Campitellib, Stefano Cordinera, Mridul Gautamb, Alessandro Mariania, Vincenzo Mulonea,* a Dipartimento di Ingegneria
Trang 1Biomass fueling of a SOFC by integrated gasifier: Study of the
Gennaro Campitellib, Stefano Cordinera, Mridul Gautamb, Alessandro Mariania,
Vincenzo Mulonea,*
a
Dipartimento di Ingegneria Industriale, Universita` di Roma “Tor Vergata” via del Politecnico 1, 00133 Roma, Italy
bMechanical and Aerospace Engineering, West Virginia University ESB, Evansdale Drive, Morgantown, WV 26506-6106, USA
a r t i c l e i n f o
Article history:
Received 14 May 2012
Received in revised form
21 September 2012
Accepted 3 October 2012
Available online 30 October 2012
Keywords:
Solid oxide fuel cells
Biomass gasification
Fuel cell modeling
Thermal integration
a b s t r a c t
Biomass gasification can be efficiently integrated with Solid Oxide Fuel Cells (SOFCs) to properly deploy the energy content of this renewable source and increasing the ratio of electric to thermal converted energy The key objective of this work is to analyze in
a systematic and wide process the integration of a biomass gasifier process with the SOFC operation In particular the work aims at identifying the role of SOFC H2utilization as
a basic parameter to maximize the system output and avoid gasifier bad operation issues such as tar production and carbon deposition An efficient simulation framework is used to that purpose allowing for a detailed analysis of the influence of key driving parameters The performance of the integrated system is thoroughly analyzed in the range of 1e2 kW electric power by also varying the input biomass characteristics in terms of Moisture Content (MC) Results show how a variation of the SOFC H2utilization, a parameter whose effects are also correlated with the gasifier air requirement, affects electrical power output also depending on the biomass Moisture Content
Copyrightª 2012, Hydrogen Energy Publications, LLC Published by Elsevier Ltd All rights
reserved
1 Introduction
Power production from biomass may represent a significant
way to limit CO2emissions and deploy the local availability of
energy sources Biomass is, in fact, the fourth largest source of
energy in the world, accounting for 15% of world’s primary
energy consumption; this number is consistently higher,
reaching 38%, in the developing countries[1,2] However, due
to the limited volumetric energy content, the high potential of
biomass is more suitable within the Distributed Generation
(DG) power production concept that foresees the use of small
size (from few to about 500 kWe) power plants In such
a characteristic size, the combined generation of heat and
power is of utmost importance to guarantee the best fuel exploitation, and reach the maximum as possible total effi-ciency htot¼ helþ hthwhich accounts for both thermal and electric use of the converted energy, commonly in excess of 0.8e0.9 Depending on the specific application, different values of the electrical to thermal power ratio may correspond
to same total efficiencies htot, depending on both technology and system characteristic power size
Several integration strategies are currently available, allowing for the use of biomass for energy conversion, which may either be based on traditional technology (i.e micro-turbines, internal combustion engines or Organic Rankine Cycles ORC turbines) having electric efficiency in the range of
5 Presented at the ASME 4th European Fuel Cell Technology and Applications Conference, December 14e16, 2011
* Corresponding author Mechanical Engineering Dept., University of Rome “Tor Vergata”, via del Politecnico 1, 00133 Rome, Italy Tel.: þ39 06 72597170; fax: þ39 06 2021351
E-mail address:mulone@uniroma2.it(V Mulone)
Available online at www.sciencedirect.com
journal home page: www.elsevier.com/loca te/he
0360-3199/$e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC Published by Elsevier Ltd All rights reserved
http://dx.doi.org/10.1016/j.ijhydene.2012.10.012
Trang 215e20%[3,4] or use H2as an energy vector [5] Among the
thermo-chemical conversion technologies, biomass
gasifica-tion is a popular opgasifica-tion giving high overall electric efficiency
[2,6e8], especially as far as more complicated layouts are
concerned, such as microturbine-fuel cell hybrid [9] An
accurate system integration would nevertheless be required
in this case to guarantee a better deployment of the biomass
energy potential
The combination of highly efficient Solid Oxide Fuel Cells
(SOFCs) and gasification systems is an effective technology for
reaching both energy and economic feasibility of combined
heat and power production [10] In this case, in fact, the
syngas can directly feed the fuel cell, which is tolerant to CO
and able to directly or indirectly convert the CH4contained in
the producer gas In the integrated biomass gasifiere SOFC
system, special attention must be given to the three main
system components (gasifier, gas processing unit and fuel cell)
while still maintaining the system complexity minimum as
possible not compromising the long-term stability of the fuel
cell at the same time[11]
The coupling of SOFCs to gasifiers is discussed in several
studies[12e16] Experimental and modeling activities are
re-ported aiming at identifying the most efficient configuration
of key operating parameters on the performances of the
different components and at analyzing the influence of syngas
composition on the fuel cell performance[17,18]
From the modeling point of view, some papers describe the
effect of basic syngas fueling on cell performance by using a
0-D representation [16], whereas the influence of Fuel Cell
design parameter is also evaluated in detail (either by
multi-dimensional modeling or direct experimental testing) to
understand the difference in operation when feeding the cell
with hydrogen or methane or with syngas characterized by
different composition
Among the others, thermal integration of the different
components is a key aspect in the development of sustainable
solutions as it potentially allows for a considerable increase of
electric efficiency hel[19,20] Depending on the design,
gasifi-cation may be autothermal or allothermal whereas the SOFC
may be operated under different fuel utilization conditions To
the aim of sustaining the gasifier operation and producing
syngas characterized by high energy content (e.g Lower
Heating Valuee LHV) as well as a more favorable H2/CO ratio
[19e22], system optimal configuration may be different from
what is required for the single component Moreover, solid
carbon deposition issues may arise thus requiring the use of
steam to avoid the rapid clogging of FC channels This issue
may be the target of a specific optimization strategy while
operating with standard biomass feedstock, which may
generally be characterized by high Moisture Content (MC) and
contain enough water to face with the cited issues The correct
use of this water content is nevertheless a function of required
energy for steam production to sustain the steam reforming
regime This energy may be recovered from the SOFC off-gas
cooling but requires specific design and control of the
inte-grated systems However, in the cited papers, a key parameter,
such as fuel utilization, and more specifically the utilization of
H2that is the main gaseous reactant responsible of
electro-chemical reactions especially at average (e.g 0.6e0.7 V) voltage
operating conditions[23], is imposed as a constant[24], or the
impact of its variation is not even discussed[21] In this paper the role of this parameter is studied from the point of view of system efficiency and has been varied accordingly in order to evaluate the combined effects on fuel cell efficiency (which may increase if more H2 is available to the electrochemical reaction) and the corresponding losses at a system level Special focus has also been given to the analysis of the varia-tion of feedstock characteristics, in terms of biomass MC, that may easily change during normal operation for biomass fueled power production systems
To the presented aim, a 0-D model describing the power plant, including a gasifier, a SOFC and all the thermal exchangers providing integration heat, has been developed and implemented under the Matlab/Simulink environment The model has been used to make a wide screening of oper-ating conditions and identify the most efficient ones The details of the used models for the single components and the overall system are given in the next section whereas results are illustrated in a specific section
2 SOFC-gasifier integrated 0-D model
A numerical model, that comprises two main sub-modules, has been implemented to represent the behavior of a SOFC coupled to an integrated downdraft gasifier In fact, updraft and downdraft technologies are usually preferred to fluidized bed for small size applications especially for temperature control issues[25] The downdraft technology was moreover selected for the low tar yield, that is particularly important for SOFC fueling [26,27] According to the system schematic provided inFig 1, the downdraft gasifier is directly fed with wet biomass, and is thermally integrated by the SOFC off-gases after combustion in the burner The system includes another heat exchanger to pre-heat the SOFC feeding air The 0-D approach has been selected, as it is characterized by affordable computational timings for optimization analyses The overall model has been implemented in Matlab-Simulink computational environment Synthetic details of the two modules are given in the following two sub-sections
This module predicts the syngas chemical composition downstream of the gasifier in terms of gaseous species mass
Fig 1e Schematic of the system: SOFC fed by integrated biomass gasifier
Trang 3fraction and overall HHV as functions of the inlet MC of the
woody biomass
Chemical equilibrium is assumed for all reactions and
implies that pyrolysis products are all consumed into the
reduction zone before they leave it The global gasification
reaction taken into account [28]may be written for woody
material (CH1.44O0.66) as
CH1:44O0:66þ wH2Oþ mO2þ 3:76mN2/x1H2þ x2COþ x3CO2
þ x4H2Oþ x5CH4þ 3:76mN2
(1) where w is the amount of water per mol wood, related to the
biomass MC as shown below
w¼18ð1 MCÞ24MC
m is the amount of oxygen per mol wood, and xiare the molar
fractions of gaseous product unknowns
The energy balance may be written instead as follows:
dHfwoodþ wdHH 2 OðlÞþ dHint¼ x1dHH 2þ x2dHCOþ x3dHCO 2
þ x4dHH 2 OðvapÞþ x5dHCH 4
where dHfwoodis the wood standard enthalpy of formation,
while dHintis the integration heat term given by the burner
under the assumption of exploiting the residual heating value
of the syngas composed by H2, CO and CH4 The energy
balance, along with elemental balances, equilibrium
constants, and further chemical reactions such as methane
formation and water gas shift reactions, constitute a three
non-linear equation set
Experimental data[29]in the case of adiabatic condition
(dHint¼ 0) have been used to validate this module The effect
of the input biomass MC on the syngas composition in terms
of dry basis volume fractions is shown in Fig 2 First, CO
decreases with MC increment as expected Hydrogen and
carbon dioxide increase with MC, while methane fraction is
negligible (1%) High N2fraction is observed all over the entire
MC range[30]
Table 1 shows the validation of this module against experimental data[29] This has been performed in terms of syngas heating value for different operating temperatures and
MC equal to 0.2 For a wide temperature range where the SOFC
is expected to operate, the heating value leads to appreciable predictive accuracy The availability of enough residence time (that is a function of the gasifier specific design) and temper-ature, allow chemical reactions to almost reach equilibrium, thus giving relatively low errors overall In summary, this module is capable of evaluating the amount of air required for gasification of the selected biomass and the syngas composi-tion by only using biomass water content w, operating temperature and integration heat (dHint) as input data
The SOFC module, that is based as mentioned on a 0-D approach, is capable of representing a planar stack under steady state operating conditions[19,20] The main assump-tions are that the cell is isothermal, H2is the only gaseous species participating to electrochemical reactions, and the SOFC H2fuel utilization is a constant input parameter Water Gas Shift (WGS) reaction is also taken into account to calculate further H2production via H2O in the SOFC anode This has been modeled at its equilibrium state with an operating temperature equal to the SOFC, and atmospheric pressure The equilibrium constant of reaction is evaluated by van’t Hoff isothermal assumption and according to reactants and products molar fractions
Fig 2e MC effect on outlet gasifier composition (T [ 1073 K)
Table 1e Volumetric heating value [MJ/m3] as a function
of temperature [K]: experimental-numerical comparison
in the case of MC[ 0.2
1023 4.812 4.9 1.8
1073 4.739 4.8 1.3
1123 4.638 4.5 3.0
1173 4.517 4.6 1.8
Trang 4Kp¼ Kc¼ ½CO2½H2
½CO½H2O
Since residual moles may be computed as follows:
nCO res¼ nCO initð1 a1Þ
nH 2 O res¼ nH 2 O initð1 a2Þ
moles of H2yielded with the WGS are
nH 2¼ nCO inita1¼ nH 2 O inita2
and the equilibrium constant can be expressed as
KC¼ nCO inita2
nH 2 O initð1 a1 a2þ a1a2Þ
By solving this equation set and applying Hesse’s law, the
new syngas composition available for the SOFC is obtained as
well as enthalpy balance of the WGS reaction
The cell voltage is defined as
where EMF is the open circuit ideal voltage according to the
Gibbs free energy at the imposed temperature, whereas
modeling of the three losses is implemented via electrolyte
resistance and thickness (DEOHM), Tafel equation (DEACT) and
as a function of H2partial pressure via the SOFC H2utilization
(DECONC)[19,20]
Ohmic losses are computed as follows:
DEOHM¼ RE$Jt
where Jtis the current density and REis the electrolyte
resis-tance defined as
RE¼le
se
with leand seequal to electrolyte thickness and conductivity
respectively The last one is evaluated by using the formula
se¼ b1eb2 =T½U$m1
where b1and b2are coefficients describing the YSZ electrolyte
behavior[22,31e35]
Overpotential losses have been modeled for anode and
cathode according to Tafel’s law
DEACT¼ ACAT$ln
Jt
J0 CAT
þ AAN$ln
Jt
J0 AN
where the cathode gives the main contribution, and ACAT/AAN
are defined as
ACAT¼ RT
2FgCAT
AAN¼2FgRT
AN
Exchange current density values J0are affected by molar
fractions of species evolving at the anode and cathode
respectively and thus the fuel utilization mffactor plays a key
role for their evaluation
J ¼ 1$108x x e100$103=RT
A=m2
J0 CAT¼ 5:5$108 xO 2
0:25
e120$103=RT
A=m2 with
xO 2¼ l mf
l
0:21 mf
¼ nH
2 , init
1 mf
_mTOT
xH 2 O¼ nH
2 ,
O initþ nH
2 , init$mf_mTOT
In the presented equations l is the air fuel ratio, which is usually in the range 2< l < 4[36,37]for syngas fueled SOFCs, while _mTOTis the minimum syngas molar flow rate at the inlet SOFC section to allow for H2oxidation n
H 2 , initand n
H 2 ,
O initare the molar flow rates, for H2and H2O respectively
Concentration losses due to mass transfer limitations are taken into account according to the 0-D modeling approach and the fuel utilization The whole phenomenon has been represented by using two different formulas
DECONC¼RT2F$ln PH 2 init=PH 2 final
DECONC¼ m$en$J t
The first equation better represents concentration losses when current density is low and the major part of the H2is still available The second equation instead is empirical and represents fairly well the partial pressure decrease effect at high current density due to mass transfer limits
Data of a SOFC modeled with 1-D approach and fed with pure H2[21]have been used for validation Voltage and current density at different temperatures and mf’s have been inves-tigatedand are provided inTable 2 It can be observed how the maximum power density [W/m2] output at isothermal condition (T¼ 1073 K) matches reasonably well with the lower
mfcases, while, for fixed mfcases (at 0.8), better results have been obtained at lower operating temperatures
In summary, given the satisfactory agreement with a 1-D model, the SOFC 0-D module here presented has been considered capable of evaluating cell voltage, current density and H2, CO, CH4, residual molar fractions for a unit cell unit in planar configuration once given as inputs the electrolyte characteristics, operating temperature and syngas character-istics in terms of species molar fractions
Table 2e Comparison of the 0-D SOFC model (current work) with a literature available [21] 1-D SOFC model in terms of power density [W/m2] as a function of
temperatureT [K] (at fuel utilization[0.8) and fuel utilization [$] (at temperature[1073K)
1023 578.4 600 3.6
1123 1602 1720 6.92
1223 2997 3250 7.78
mf 0-D model 1-D model Error % 0.85 759.8 750 1.29 0.95 396.3 430 7.84
Trang 52.3 Algorithm of the numerical model
The presented numerical model is composed by several
modules interacting until the steady Gasifier-Solid Oxide Fuel
Cell energy system operating point is attained A flow diagram
of the numerical model is provided inFig 3to better
under-stand the solution algorithm
Fig 3 shows how air pre-heating and reforming require
a certain amount of heat which has to be assured for the
energy system sustainability If these two requirements are
not satisfied, the input setting imposed is considered by the
model as invalid to run the whole energy system Conversely,
with an input setting which yields extra heat at the burner, the
integration process at the gasifier is considered physically
consistent
3 Analysis of results
The model has been applied to the analysis of the main
performance parameters of the SOFC and the integrated
system, when fueled with woody biomass The influence of
biomass MC by varying fuel cell operating conditions has been
analyzed to define the best compromise among power density
and efficiency of SOFC and thermal balance of the system
This is in fact directly influenced by the following operating
aspects:
The gasifier regime, that is primarily defined, given the
operating temperature, by the air mass flow rate, whose
variation has a high impact on syngas composition and
yield;
The SOFC H2fuel utilization, which is an operating
param-eter linked to the electrochemical exploitation of the cell
active area
The SOFC active area has been considered equal to 1 m2,
while the woody biomass inlet flow has been set to about
1 kg/h (dry basis) and constant, corresponding to an input thermal power in the range of 4.5 kWth The SOFC operating voltage has been controlled in the code to lie in the range 0.6e0.65 V, that gives an optimal compromise between cell power density and efficiency
A first comparison between the performance of the auto-thermal and auto-thermally integrated layouts has been analyzed
by means of the system model at biomass MC¼ 0.4 Results are given inTable 3, confirming the increase in efficiency of the integrated system (hsys,integ¼ 37.7% vs hsys,autoth¼ 24.7%) The higher system performance has been achieved despite the lower total fuel utilization, while the SOFC efficiency has been kept in the optimal range of 50% System efficiency increase has then been observed for the better gasifier oper-ating regime, which allows for a different syngas composition, that is reported inTable 3 A remarkable increase in terms of
H2 concentration is in fact obtained with the integrated system (48.5% vs 17.8% on a volume basis) This has been achieved by a drop in gasifier air flow rate, that leads to both the decrease in N2 concentration, i.e no related transport losses occur in the SOFC, and to a lower O2flow rate (almost
0 vs 0.69 kg/h) that is possible as heat is provided by the combustion of SOFC off-gas, according to the schematic of
Fig 1 This basically means that the gasifier, due to the high biomass MC (0.4), is operating under “steam reforming like” regime with no O2, that would otherwise be unfeasible without the thermal integration Thus, the drop in SOFC fuel utilization is not indicative of fuel waste, but rather of better gasifier operation that eventually leads to higher system efficiency
Three different MCs have then been further analyzed for the integrated system: 0.1, 0.3 and 0.5 by further assuming constant SOFC H2utilization (0.8) The effect of MC is such that
a higher electric power is delivered by the SOFC going from
MC¼ 0.1 to MC ¼ 0.3 conditions, meaning that fuel utilization
is enough to thermally sustain the gasifier and allowing for
a decrease in air (i.e O2) flow rate (red curve inFig 4) The situation radically changes going from MC¼ 0.3 to MC ¼ 0.5 conditions: in fact, MC is so high that the integration heat
dHint is not enough to support the gasifier via the off-gas combustion This is also linked to the SOFC H2 utilization, that has been thus identified as primarily important
Fig 3e Whole numerical model flow diagram
Table 3e Comparison of system performance parameters between thermally integrated and autothermal gasifiers
System performance parameter
Integrated Autothermal
Current [A/m2] 3045 2129 Voltage [V] 0.64 0.60
hSOFC 51.3 48.0
hSYS 37.7 24.7
O2[kg/h] 0.004 0.690
H2[%mol] 48.5 17.8
CO [%mol] 28.3 9.4
CO2[%mol] 9.7 14.0
H2O [%mol] 12.4 19.8
CH4[%mol] 0.7 0.1
N2[%mol] 0.4 39.0
Trang 6parameter Results are reported inFig 5, where the effect of
the variation of SOFC H2utilization has been added It may be
observed that, as a general trend, the lower SOFC H2
utiliza-tion operating condiutiliza-tions have to be avoided In fact, despite
the theoretically high availability of integration heat, a high
amount of air (O2flow rate in the Figure) is required to let the
SOFC operate at the selected operating voltage This also leads
to a poor exploitation of the SOFC off-gas that is evident by the
analysis ofFig 6, where dHintis reported as a function of SOFC
H2utilization and MC dHintrepresents the amount of the
off-gases enthalpy that is actually utilized in the biomass gasifier
It thus appears that low energy is required by the gasifier
to operate if air flow is sufficiently high, as in the already
commented operating conditions with low SOFC H2utilization
and MC
An increase in the electric power output is observable with
H2fuel utilization at any of the proposed MCs This is directly
related to the decrease in required O2flow rate (againFig 5) At
the same time, the integration heat dHint (again Fig 6) is
increased, as an opposite effect to the decrease of O2flow rate:
in other words, the gasifier is moving more toward a “steam
reforming like” operating regime However, a peak in dHintis
observed, more evident for the higher MC values (e.g
MC¼ 0.5), as far as higher than 0.8 SOFC H2utilizations are
approached That peak indicates that a limit is achieved in
terms of thermal integration potential that is due to two
concurrent phenomena:
1) The heating value presented by the off-gas is decreased by
the higher exploitation of the SOFC at higher SOFC H2
utilizations
2) Higher MCs allow the gasifier to produce syngas
charac-terized by higher H2/CO ratio further giving much more
favorable water gas shift behavior into the SOFC [19,20]
This is key to both have the SOFC operating much closer to
pure H2 fueling operating conditions, as well as having
much lower off-gas waste
3) The peak in power is also obtained at the minimum in
terms of O2request, further testifying that criteria based on
the gasifier air requirement are practically equivalent to optimize the SOFC H2utilization
InFig 7, the plot of current density is given, whose trend is exactly similar to the power curve plotted inFig 5 On the other hand efficiency, again in Fig 7, presents a slightly different trend since the biomass LHV (being the denominator
of efficiency) is also affected by MC Thus, the difference between maximum efficiencies at 0.5MC and at 0.3MC is higher than the same difference in terms of power
Finally, the link between air (O2) requested by the gasifier and integration heat (dHint) is extremely important, and the capability of the gasifier to work with minimum air request has to be sought, along with the control of SOFC operating voltage Among the cases reported inFig 5, optimal operating conditions in terms of SOFC H2utilization or O2requested flow rate may be selected depending on the specific biomass MC, further confirming that high MC operating conditions should
be preferred in terms of electric power maximization
Fig 4e Electric power of FC (kW), H2production (%) and
required O2mass flow (m3/h) as functions of MC at
constant SOFC H fuel utilization equal to 0.8
Fig 5e Electric power (kW) and required O2mass flow (m3/h)
as functions of SOFC H2fuel utilization and biomass MC
Fig 6e Efficiency (%) and current density (A/m2) as functions of SOFC H fuel utilization and biomass MC
Trang 74 Conclusions
The operation of a SOFC fed by an integrated biomass gasifier
has been demonstrated to highly depend on cell operating
conditions, and specifically on SOFC H2utilization, that is also
correlated to gasifier air requirement, and the biomass
Mois-ture Content (MC) In fact, gasifier operation, and more
specifically its steam, autothermal or intermediate operating
regime, determines a great difference in terms of SOFC flow
rate and syngas composition
Obtained results allow to draw the following main
conclusions:
At constant cell operating voltage and MC operating
condi-tions, the thermally integrated system gives much better
performance in terms of power output and system electric
efficiency (37.7% vs 24.7%) The MC (i.e available H2O) and
integration heat are thus used to operate the gasifier more in
the steam operating conditions rather than autothermal
That is also indicated by the almost zero requirement in
terms of gasifier air flow rate
System performances are highly dependent on SOFC H2
utilization, that has a direct impact on gasifier operating
conditions via its thermal integration by the combustion of
the SOFC off-gases
SOFC H2 utilization influences the system performance
mainly by two effects: air flow rate is more important in the
low SOFC H2utilization end, where the selected cell voltage
is achievable only with high air flow rate; the limitation in
terms of integration heat dHintis instead more evident in
the higher SOFC H2utilization end where air is required to
thermally sustain the gasifier
Nomenclature
dH enthalpy variation
E cell voltage, overpotentials
F Faraday coefficient
K equilibrium constant
l electrolyte thickness
_n molar flow rate
R electrolyte resistance
x molar coefficients Greek symbols
b thermal conductivity coefficients
l air fuel ratio
s thermal conductivity Subscripts and superscripts
c, conc concentration
r e f e r e n c e s
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