Prcc..dinpt oJ tnt.ndional CoaIet.n . On Ci.di.at and aioprcc.ts FnSineering
27' 2f A,eu! 2OOJ Univ.t\iti Malavio Sobah Kota Kinabotu
Simulationand sensitivity Analysisof AutothermalReforming(ATR) for Mobile
Fuel Cell Application
Arshad Ahmadl
Norazana lbrahim'
tDepahnent of ChemicalEngineeing
Uniwrsiti TeknoloqiMalaysia, 813l0 WM Skudai'Johor' Malavsia
utn n,
Tel: +60-7-55 35610,Faa: +60-7-558'1463,Email: atshad@Jkkksa
zlnboratory of PrccessControLDepanmentof ChemicalEngineerinS
Univeniti TeknologiMalqsia,81310 UTM Skudai'Joho4 Malavsia
Tel: +60'7-55 3s858,E /1il: n-4na78@vahoo.con
Abstract
Autothermal Refonmng (ATR) is one of the latest
technologiesfor fuel reforming to pmduce hyalrcgenfor
fuel cell automotiveapplication ATR is a combinationof
an endothermic and exothermic process to give th€
maximum hydrogen Foductivity. It integntes the heat
effecl of the panial oxidation and steam teforming
reacdonsby feeding the fuel, water and air or oxygen into
the reactor.This processis carded out in the presenceof a
catalvst.which controls the reaction pathwaysand ihereby
.tetem;ne the relative extents of oxidation and steam
reforming reaction.In this paper, the developmentof ATR
MATLAB/SMULINK
using
model
simulation
environment is presented. Based on this model' the
sensitivity analysisof the Focess is Performed-fie r€sults
of this studyarediscussed.
Keywords:
jr]
refornung:hydrogenproductiv
A urorhermal
MATLAB/SIMULINK; sensitivity analysis
Introduction
Fuel cell power systemhas receivedincreasedattentionfm
tansportation applicationsin recent years becauseof th€ir
potential for higb efficiency and lower emissions [l-2].
Fuel cells aft an electrocherical system that converts
chemical energy directly into electdcity by Fomohng a
chemical reactionbetweentwo reactantgases.Therc arc a
variety of fuel cells systemsfor a differcnt application is
under development. These include proton-exchange
memblaBefuel cels (PBMFC), alkaline fuel cells (AFC)'
phosphoricacid fuel cells (PAFC), molt€n carbonatefuel
cells O4CFC) and solid oxide fuel cells (SOFC) [3]
Among these,PEMFC systemsare being studied in this
PEMFC generaieselectric power from air and hydrogenor
from a hydrogen-richgas. Water and waste beat arc the
8't6
only by-producb.Hydrogen-rich gascan be producedftom
conventional traosporiation fuels via various reforming
technologies.There are three major reforming technoiogies
which are Steamreforming (SR), partial oxidation (PO) and
aubthennal reforming, all consistingof similar steps.First'
the fuel is vaporized.It is desiredto maximizethe hydrogen
contentwhile decreasingthe carbonmonoxideandm€thane
formation.Then this gasmixture is further processedin the
shift reactor. ln this reactor, carbon monoxide is reacted
with steamreforming to pmduce additional hydrogen by
the water-gas shift reaction. The remaidng ca$on
monoxide can be further €onvettedinto ca$on dioxide by
selection oxidation for hydrogen purification The
hyaLogen-richfuel containingcarbonmonoxideat below 10
ppm levels is readyto be fed [4].
Steam refoming shows the highest hydrogen Foduction
efficiencies. However, the required heat input due to
endoihermicr€actionsis consideredas a major drawback
for autoriotive applications. Partial oxidation needs
extemal cooling. Aulothermal reforming (ATR) prornises
better dyoamic response€omparing to tbese two reaction
An ATR system is a coupling of endothermic and
exothemic prccess to give the maximum hycirogen
productivity. The auioihermal reaction integratestbe heat
effect of the partial oxidaiion which is cossisting of the
sutsstoichiometric oxidatlon of methane, with steam
reforming [51. Under such conditions,the reactionmixtue
can be usedas a heating/coolingmediumand lhe reactoras
a heat exchanger,all in one compact unit insaeadof a
networkof reactorsafld heatexchanger[6].
This study focused on numerical simulation of the ATR
wilh steady staE and d).nanuc model by using
MATLAB/SIMULINK environment.Basedon this model,
the sensitivily analysisis perfomed to determineprocess
constraints. In the ATR fuel processor, vaponzec!
bydrocarbon fuel, oxygen and water (steam) are fed at
controlled conditions to the reactor to produce the
8as mixture in an aulothermalwaY. The
gas mixture containing the desired hy&ogen
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Prou.dines of lntemrioial ConJ.tuMeOn Chaniat ud BioprocessEnsiiedins
27x -2* Ausue2OO3. Univ.rtiti Mataysiasdbnh,Kota Kinabalu
must be processedfurther to conven carbon monoxide to
carbondioxide. The hydrogen-richgashasto b€ cool€dand
humidified to desiredfuel cell inlet conditions,The quality
of the raw rcformate (i.e. CHa, Or, CO, CO, and HrO
contents)is stronglyaffectedby the reforming condition.
lr shouldbe noredthat Lheconcenbalion
of the Hr in the
reformateinfluencesthe p€rformanceof tbe fu€l cell stack.
Higher hydrogen concentrations show better firel cell
perfomance. In gen€ral. CO content in th€ product
hydrogenhasto be below l0 ppm in order to be usedas the
anodegas for PEMFC [7]. In addition, if purc hydrogenis
used in the feed stream,there is no power degadation of
fuel cell stackand hence,increasingthe life time of the fuel
cell. Another advantageof using pure hydrogenis that fuel
cell stackcanrun dead-end.
It runswihout thebleed-outof
th€ anode gas from th€ stack, and hence maximizes the
hydrogenutilizationandmin;mizesthe prcssurclossin the
stack due to the lower total gas flow through the stack. In
line with ahesereasons,the use of hydrogenpurifier
downsreamof the ATR systemis desimble.However,a
detailed hydrogen purification model is not part of this
paperbut is fonhcoming.
DeYelopmentof reactor model
Solid phase
.",,"='
,,j ! ft(,'*Ll+)).
(3)
n,a"@,
=o 1ay
-r,)+(r-e
aru,)p,n,,,
")I,(Gas-phaseboundaryconditions
u=0
(5)
Ct=Ci.rt-r;
Solid-phaseboundaryconditions
d f c..,l_^
E= 0
dtlpt
( 6)
)
€=r p,?-Ll+l
.
pr
ro ae\
=k,(c,c ; . , )( 7 )
)p.,
The simulation of the autothermalreacrionis basedon the
following set of differential equationst9l.
ContinuityEqratiotLe
Model assumption
In view of opemtion characteristicof the adiabaticfix€dbed reactorsystemfor tuel cell application, the following
assumptionsare introduced:
gradienboDlyoccursin
L Concentration
and Lemperature
the axial direction.
2. The catalystparticle is assumedto be isothermalard the
main transpof resistanceinside the catalysl pellet is due
to masstransfer,even in the cas€of highly exothermic
reaction[8].
3. The transport mechanism in the axial direction is
considered
to beof theplug-flowtype.
Mathernatical model
A one-dimensiomlheterogeneousmdel is chosenin this
work to simulatea tubular fixed-bedrcactor,The massand
en€rgyequationsfor the bulk and gas phaseas well as the
corresponding
initialandboundaryconditionsaredescribed
below[8]:
(8)
?= 9r , "+ 4, r , + q, r . )
+=#b,",")
(e)
t;-=ffitn'u-n",t
(10)
.lx @,
(1r)
a"-
=
p,Ct
,
i;r't
+n
"'
'\
+ 4'r' )
Energyequation:
*t
*= *ftr,0,',e
(t2)
For the prcssuredrop, the following equationis used:
(tjr
!g! =_fp,"i
Oar phare
," *( +) . r "' ( c ,- c ;- ' ) h;o..\r - r, )= o
"
ISBNr983-2643-15-5
(1)
The effectivenessfactors,I aretakenfrom an averagevalue
' basedupod a numberof off-line pellet simulationsfor the
variouseffectivenessfacio$ [9].
(2)
877
Ptu...aiqt
ReactioDkinetics
In the modelling of autothermalreforming of methane,it is
necessar/ to combine all the rale equations for the tolal
steamreformingand watergasshift reactions
combustion,
in the calculations.ln this paper, the intrinsic reforming
modelsproposedby Xu and FromentFol are adoptedas
presenred
below.Theseauthorsderivedthe intrinsicrate
equationsfor the steam reforming of meftane on a
NiA-,lgAlrorcatalyst.
Totaloxidaiion
(14)
CHA+20,-CO,+2H,O
r = .
'
k"P,,'Po.
( ls)
k nPc a"Pot
*,
\n x.;p.,,- x;:,p",| F; x;;;;;
x:;S
SteamRefonning(Co production)
(16)
CH4+HzO€CO+3H2
k{ / P';tr@"",p",o
- p',"p- tK".,
(l + K apc n + K , apH 2 +K c B ,p a , + Kp z o p tto lp a )'
(17)
Warer-gasshift reaction
(18)
CO +H"O c1 CO1+H.
k i I po' ( p- p o ,o '
(l+ Kap@ + Ka*,2+
Kdapcr,
Resultsand Discussion
The reacbr and caralysldimensions.
as well as operatjnq
condilionsuselo opemlelhe fixed-bedreactorlor hydroAe;
tor fuel cells produclionis shown in Table t. In fiis
application,ihe reacbr is operatedat atmosphericpressure
with methaneandoxygenas a fe€dstock.
Besides,wateris
addedas reactantat the reactor inler ro increasehydrogen
formation and to suppresscoke deposition.
Table I - Reactordinensions,catabst dimensionsand
operatingconditions
Operatlng Conditions
Fuel Cell
Reactor
Diam€terof reactor,d, (m)
Length of reactor,l, (m)
Temperaturc(K)
Pressure,atm (Pdd)
o.4
0.5
830
Catal!st
Catalystused
Metal content(wt 9,)
Metal surfacearea(m'?g-r)
Density,p" (ksm-')
Feed Composition
CHIOz
HrO/CIt4
Oxygensources
I
Ni&lgAlzO3
t5.2
4.1
18?0
2.O
2.0
+ KH\opEb I pE)'
SteamRefonning(CO,productjon)
CO,+4H,
(20\
kl"I p'itroe.p'zB,a
- pLpco,tK
(I+ K @p@+ K r r pp 2 +K@ ,p u t + K o z p
o o ,o l p o )'
(2t,
Simulation Studies
Fouowing the developmentof the mathematicalmodel of
the syslem, simulation studies were implementedin
MATLAB/SIMULINK environm€nt.Sensitivityanalyses
were canied oul to evaluate lhe influence of some key
processvariablesoo tie ovefall systemperformance, The
reactor was assumedadiabatic and the fuel gas mixture
(Methane/steanvoxygen)
is fed into the ATR at ihe selected
T^rR The tenperarureof the exit product stream is
detemined by the extent of the endotbermic and
878
exothermic reaction in the reactor due to adiabatic
conditions.
p o ' P* ,1 K
(19)
CH 4+2H,O..
ol lnkndiaut
ConIem, . On Ch.nical ord Aopo..:,
Enskpetine
27r
2er A'sur 20AJ, Unn.^iliMatatsta Sobah.Kota Kitubaiu
The parametric sensitivity of ATR behaviour and its
performancehave beeninvestigaledfor sevemlparameters
including opemting temperaturesand kinetics parameler.
The influence of these opeB.ional parameterson the
product composition,i.e. product disrribution,depends
strongly on the thermodynamicsof the reacaions.For this
purpos€, analysis has b€en studied over the temperature
rangeof 800-1015K, feedflowratefrom 1.0-3.0m'/s and
steamto methaneratio 2.0-4.0. lt has been found that the
optimal operatingtemperaturein ATR is f0f5 K and fe€-d
flowrate of 2.0 m3/s andstean to methan€ratio is 2.0-3.0.
The rcsultsare asfollows:
1 Effect of temperature
Figwe I shows that methaneconveNion at different feed
temperatufes.It indicatesthat methaneconversionincreases
with increasingfeed temperature.It reachesabour 98.9%
conversion
at a temperanrre
of l0l5 K from 52.47%at 800
K. It hasb€enfound that after this value,a systembecomes
unstable.
2. Effect of step change in temperature on product
distribution.
ISBN:983-2643-15-5
d
Prcce.din$ of Intenatiotul confer.nce On Chehical ahd aioptucest Eneihe.ibg
27' 2y Augd 20A3. Univ.rcni MdLarsiaSdbdh,Kab Kindbal,
Figure2(a), clearly showsthat the methaneconcentrationis
decreased\rhen tempemtureis increased.This is due to the
increase
ofthe amounrof availableoxygenwhichenhances
the exorhermicreactions(CO" fonnation). It hasbeennoted
fiat lhe CO concentrationincrcaseswith lemperature.
when the temperature
becomeslarger than 900 K, high
concentradon
of hydrogenis obtainedas shownin figures
2(c) and 2(d).In contrary,figure 2(e) indicatesthat steam
{H.O, concenrralionis de{reas€dwhen temperatureis
iocreased.This is becausethe higher the operating
temperatufe,ihe more steamis fleededfor steamrefom)ing
3. Effect of step changein feed flowrate on product
distribution.
From the figure 3. the behaviourof productdistributions,
mainlyconceniradon
of CHj, CO?,H, andCO followsthe
pa[em of the step change in temp€rature.At higher
operaringtemperature,the oxygen is almost convertedand
it becomesunstable(negativevalue) as temperatureabove
1000 K and fe€d flowrate greaterthan 3.0 mr^ were used
Geefigure3(f))
2(b)
! 07
,:I
::l
e
E 0.6
;I
:il
;il
:l
2(c)
-t
Td.
(.)
Figurc I - Tenpenture pmfiIes thrc eh the rcactor with
:
;
E
! o.r5
€
2(d)
2(a)
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Pnccedius oJlntematioul Cod.r.nce On Ch.nical ana Bioptuess Eagircei,s
27" -2q Aleui2003, Univ.rsiti Malarsia Sabdh,Kota Kinabdb
R 0.16
E01
E
E 0 . 1r
0 25;
2(e)
I
E
E
6
nn. (.)
2A
3(c)
Fieu.e 2 - Effect of chansine tenperuturc on prcduct
disuibution: (a) CHa,(b) COr (c) H, (d) CO, (e) HrO, (f)
Or. Fk conditions: Feedfolt)rate I mr/s, pressure I bar,
and steamto methanentio 2.0
3 0.55
E
E '.,
+ o.j
,i 06
s(d)
3(a)
880
ISBN:983-2643-15-5
Ptuc.editss of lntemdtiotul Conferdc. On Ch.hical @d Bioprccest EneiieennB
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Besides,higher methaneconversionis obtainedupon
decreasingthe feed flowrate and increasing temperatur€.
From the thermodynamic
analysesand kinetic simulation
performed, $e optimal operating conditions of the
temperature<1015l<, HrO/CH4 ratio of 2.0-3.0,feed
flowrate of 2.0 m'/s afld ]ow pressure reforming is
favomble due to the PEMFC is operate at atmospheric
I
€ 0m
References
rin€ (3)
3(e)
a
E
::l
3A
Fisurc 3 - Efect of chansesof feed.Jlowrate on prcduct
distribution. (a) CHa,(b) Coz (c) H,. (d) CO, (e) H,o, (f)
Or. FLxconditions: TemperuturcI0l5 K, prcss rc 1 ba\
tutio 2.0.
steamto methane
Conclusion
The kinetic model of Xu and Fromenl [0] for steanmetbane reforming has been shown to be applicable to
autothermalreactoropemlion. From the simulation results,
temperaturehasa significant effect on hydrogenandca$on
monoxidelevels.At low tempera$re,methancmay not be
completelyconverred,even though the o/cH{ mtio is
greaterthan 0.5, resultiogin lower H, and CO level.
However at higher temperature the steam reforming
reactions dominate and hydrogen and carbon monoxide
level increase.It is also revealedthat, lhe CO and H,
formation is favoured at high tempemturesand Hz yield
increasingat higher rate due to the water gas shift reaction
and methane coupling reaction aa higher temperature,
ISBN:983-2643-i5-5
Robert
SitaramRamaswamy,
[1] MeenatshiSundaresan.
Moore, M., and Myron Hoffman,A. 2003. Catalytic
Bumerfor an IndirectMelhanolFuelCell VehicleFuel
Processor.
PowerSourcesl13: 19-36.
[2] Ahned, S., and Krumpelt,M. 2001. Hydrogenfrom
Hydmcalboo Fuels for Fuel Cells. Hydrogen Energy
261291-301.
[3] Anca Faur Ghenciu.2002. Review of Fuel Processing
Catalyst for Hydrogeo Produclion in PBM Fuel Cell
System.SolidState& MaterialsScience6: 389-399.
[4] Ersoz,A., Olgun,H., Ozdogan,S..Gungor,C., Akgun,
F.. andTiris,M.2003. AuLolhermal
Relomingas d
Hydrocarbon Fuel Processingoption for PEM Fuel
Cell.PowerSources
5244:l-9.
(51 Reni, S., Calogero, G.. and S.Cavallaro.2000.
HydrogeoProductionftom MethaneThroughCatalytic
PanialOxidalionReaclions.
PowerSources8?:28-38.
J6l Grieorios Kolios, Jorg Frauhanmer and Gerhan
Eigenberger. 2002. Efficient Reactor Concepl for
Couplingof Endothermicand ExothermicReactions.
Science
57: l50l-1510.
Jaesung
Han IL-Su kim and Keun-SeobChoi. 2002.
Pl
High Purity of Hydrogen Generator for On-Sile
Hydrogen Produclion.HydrogenEnergy 27: 10431047.
D€
Sm€t,C.R.H., De Croon,M.H.J.M.,Berger,R.J.,
[8]
Marin, C.8., and Schouten,J.C. 2001. Design of
Adiabatic Fixed-BedReacrorsfor the Partial Oxidaiion
of Methan€ to Synthesis Gas: Application to
Productionof Methanol and Hydrogen-for-Fuel-Ce1ls.
Science56:4849-4861.
[9] Ann De Croote, M., and Gilbert Froment, F. 1996.
Simulation of the Catalytic Partial Oxidation of
Methaneto Synthesis
Gas.AppliedCatalyst138:245264.
tl0lxu, J., and Froment, G.F. 1989. M€.hane Steam
Re{orming, Methanation and Water-gas shift: I
IntrinsicKinetics.A.LCh.E.Journal35: 88-96.
8 81
PraceedinssoJlntemtiovl Cdfercnc. On ChenicaLMd BioPrc*ss Eneineeing
2f - 2q Ausd 2A0J, Oniwriti Maldrsia Sabah.Kota Kinobalu
Nomenclatrrl€
a,,,
massflow velocity,kgm,'?s-l
superficial
4,
e\rernalpeilelsufaceareaperuniLreacror
,4,
volume,mi'zm,r
pre-exponenlialfactor'reactiondependent
c/,
pressure,
J kg_'K''
sp€cificheatat constant
C,
molar concentrationof speciesi, mol m;l
q
lratronofspeciesi
inlra parliclemolarconcen
q.
i. at theextemal
of species
molarconcentration
pelletsurface,mol mtl
4
m,
reactordiameter,
D,.t
effectivediffusion coefficient of speciesA i
m, ml''1s'r
catalYst,
I
gas-lo-solid
healtransfercoefficient,w mi'?K
l"
r€ac.orlength,m
pr
i, bar
ofcompoBent
Panialpressure
P,,
total Pressure,bar
ri
mteof reactioni, mol k&;'s_'
p€llet radius,m
&
netcataiylicprodudionmleofspeciesi. peruniL
Ts
catalystmass,mol kg;'i'
gas-phasetempemture,K
T"
solid tempemhlre,K
z,
AH:
axial reactorco-ordinate,m
standardadsorptionenthalpy,J mof'
i. J mol '
-+H heatof formationof species
€R
voidfractionof packing,mgm;l
gas-to-solidmasstansfer coefficient, mg3mi'?sr
a
dimensionlesspelletco-ordinate
reactionrale constaotof feactioni
P1
fluid density,kg nl-3
equilibrium constantof reactioni
Ps
gasdensiry,kg mr'r
4
adsorptionconsrantfor componenti barr
4j
effectivenessfacaorfor reactioni
Ki"
adsorptionconstantfor componenti, in
f)
crossse€tionalofreactor m2
xrr.
convenion of melhane(%)
x*.
conversionof methaneto CO, (%)
ks
combustionreacaion,bar '
882
ISBN:983-2643-15-5