SOFC Modeling Considering Internal Reforming by …...SOFC Modeling Considering Internal Reforming by a Global Kinetics Approach and My Research in General Martin Andersson Division

SOFC Modeling Considering Internal Reforming by a Global Kinetics Approach

and My Research in General

Martin Andersson

Division of Heat Transfer, Department of Energy Sciences, Faculty of Engineering (LTH), Lund University, Sweden

October 27th, 2009

ECS & SOFC XI Vienna 2009 (updated for group sem.) / Martin Andersson / LU / Dep. of Energy Sciences

Articles written

• M. Andersson, J. Yuan, B. Sundén, Chemical reacting transport phenomena and multiscale models for SOFCs, Proceedings of Heat Transfer 2008

• J. Yuan, G. Yang, M. Andersson, B. Sundén, Analysis of chemical reacting heat transfer in SOFCs, Proceedings of 5th European Thermal Sciences Conference, Netherlands, 2008

• J. Yuan, G. Yang, M. Andersson, B. Sundén, CFD approach for chemical reaction coupled heat transfer in SOFC channels, Proceedings of 7th International Symposium on Heat Transfer ISHT7, China, 2008

• M. Andersson, J. Yuan, B. Sundén, W. Guo Wang., LTNE approach and simulation for anode-supported SOFCs, ASME FuelCell2009-85054, USA, June 2009

• M. Andersson, J. Yuan, B. Sundén, SOFC Modeling Considering Internal Reforming by a Global Kinetics Approach, 216th ECS Meeting in Vienna -Eleventh International Symposium (SOFC-XI), Austria, October 2009

• M. Andersson, J. Yuan, B. Sundén, Review on Modeling Development for Multiscale Chemical Reactions Coupled Transport Phenomena in SOFCs, Int. J. Applied Energy, Submitted, October 2009


Agenda

• Introduction to FCs and SOFCs

• ECS/SOFC XI article– Introduction– Literature survey– Mathematical model – Results – Conclusions

• Future work


Introduction to fuel cells

• Fuel is directly converted to electrical energy, with water/heat as by-products

– No Carnot cycle limitation– Environmental friendly

• The principle dates back to 1838/39• Fuel cells are expected to be a key component in a future

sustainable energy system• Strategic niche markets will be important for commercialization


Introduction to fuel cells

Willingness To Pay (WTP) for different FC niche markets


SOFCs

• Working temperature: 500 – 1000°C• Combined with gas turbine

– Overall Efficiency: >85 % – 70% electricity

• Stationary and APUs• Internal reforming is possible

– Electrode material (YSZ and Ni) as catalyst

• Vulnerable to sulphur poisoning• Tubular or Plannar design• Electrode or Electrolyte supported


SOFCs


SOFC Modeling Considering Internal Reforming by a Global Kinetics Approach

M. Andersson, J. Yuan & B. Sundén

Martin Andersson

Division of Heat Transfer, Department of Energy Sciences, Faculty of Engineering (LTH), Lund University, Sweden

ECS/SOFC XI, ViennaOctober 9th, 2009


Introduction

• The fuel cell is the invention from the 19th century that can solve the problems of the 21st century with low energy efficiency and carbon emissions etc.

• SOFC modeling is promising:– Increase the understanding of physical phenomena– Optimizing the design– Decrease the production cost

• Strong coupling between different physical phenomena, requires multiphysical modeling.

– Mass, momentum, heat and chemical reactions are considered.

• This study focus on the effect of active surface area ratio on the steam reforming reaction.


Global SOFC reactions


Internal reforming reactions - Pre-reformer vs. int. ref.

• Pre-reformer– Needs extra added steam– One extra unit to the FC system

• Internal reforming – Increased electrical efficiency– Requirement of cell cooling decreases– Big temperature gradient close to the fuel inlet need to be avoided

• Decreased inlet temperature• Recycling of anode gas• ”New” anode material


Internal reforming reactions - reaction mechanisms

• Steam reforming reaction – Global or more detalied expression– Dependent on temperature, catalytic material, partial pressures etc.

• Water-gas shift reaction– Normally considered to be in equilibrium– (1) Global reaction mechanism in the anode only– (2) Global reaction mechanism in the anode and in the fuel channel– (3) Advanced reaction mechanism including catalytic surface reaction kinetics


Internal reforming reactions - reaction mechanisms

• Steam reforming reaction– m varies between 0.85 and 1.4– n varies between -1.25 and 1– Ea varies between 60 and 230

kJ/mol

• Water gas shift reaction– Normally assumed to be

in equilibrium

⎟⎠

⎞⎜⎝

⎛⋅

−⋅⋅⋅=

TE

ppkr an OHmCHr R

exp24

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅⋅

⋅−⋅⋅⋅=

OHCHre

HCOOHCHeqr ppK

ppppkr

24

2

24,

3

, 1

⎟⎟⎠

⎞⎜⎜⎝

⎛

⋅⋅

⋅−⋅⋅=

OHCOse

HCOCOss ppK

pppkr

2

22

,1


Mathematical model


Mathematical model - geometry


Mathematical model / COMSOL Multiphysics (FEM)

• Ability to model several physical phenomena simultaneously– The free variable in one mode can be used as input in another, for example

temperature, velocity, pressure

• Many post processing options• Define a geometry (1D, 2D, 3D)• Boundary / Interface conditions• Subdomain conditions• Time dependent / Stationary conditions


Mathematical model

• Momentum transport– Navier-Stokes eq. (gas channels)– Darcy eq. (porous electrodes)– Brinkmann term (electrode/channel interface)

• Mass transport– Maxwell-Stefan equation

• Heat transport– LTNE approach

• Conductivity in solid phase• Conductivity and convection in gas phase• Heat transfer between the phases in the porous electrodes and at the

channel walls


Mathematical model

• Fuel utilization: 80 %• Oxygen utilization: 20 %• Current density: 0.3 A/cm2

• Inlet temperature: 1100 K• 30 % pre reformed natural gas


Mathematical model

• Temperature and partial pressure dependent parameters:– Gas phase

• Density• Viscosity• Heat capacity• Thermal conductivity

– Reaction rate• Steam reforming• Water-gas shift reforming

• Temperature dependent:– Maxwell-Stefan diffusion coefficients


Mathematical model - Assumptions

• 2D• The electrochemical reactions are specified at the electrolyte/electrode

interfaces • The Knudsen diffusion term is neglected • The effects on the flow profile from the inlet length are neglected• The Nusselt number is assumed to be constant • The change in entropy and enthalpy due to the chemical reactions are

defined at constant temperature only • The thermal conductivity and heat capacity for the solid parts are defined at

constant temperature only • An average current density is defined and not calculated from local

conditions


Internal reforming reactions - Mathematical model

• Steam reforming depends on active surface area– Parameter study is preformed

• Water-gas shift reforming– In fuel channel and anode

[Klein et al, Chem. Eng. Sci. 62, 1636-1649 (2007)]

rr = SA ⋅ kr , f ⋅ pCH4 pH2O ⋅ exp−Ea , r , f

RT⎛ ⎝ ⎜

⎞ ⎠ ⎟ − kr , r ⋅ pCO pH2

3 ⋅ exp −Ea , r , rRT

⎛ ⎝ ⎜

⎞ ⎠ ⎟

⎛

⎝ ⎜

⎞

⎠ ⎟

⎟⎠⎞

⎜⎝⎛ −⋅⋅−⎟

⎠⎞

⎜⎝⎛ −⋅⋅=

RTEppk

RTEppkr rsarsfsafs HCOOHCOs

,,

22,

,,

2, expexp

k = f (T)


Parameter study - surface area ratio

SAa= 619 000 m2/m3 SAb= 619 000*10 m2/m3


Result - Steam reforming reaction rate

An increased surface area ratio means an increased reaction rate close to the fuel inlet, i.e. a faster conversion of methane.


Result - Water-gas shift reaction rate

The higest value can be found where the production of carbon monixde is high.

The water-gas shift reaction proceeds to the right due to the electrochemicalreactions.


Result - Methane mole fraction

An increased surface area means that the methane is converted faster.

The fraction difference in y-direction is due to the steam reforming reactionin the porous structure.


Result - Carbon monoxide mole fraction

The highest fraction (CO) increases as the surface area is increased.

The fraction difference in y-direction is due to the steam reforming reactionin the porous structure.


Result - Water mole fraction

The highest fraction (water) is found at the outlet.

A high surface area ratio means that water is consumed faster (reformingreactions), than generated (electrochemical reactions).


Result - Hydrogen mole fraction

A high surface area ratio means that hydrogen is generated faster (reformingreactions), than consumed (electrochemical reactions).


Conclusions

• A CFD approach is developed and implemented to:– Analyze physical phenomena in an anode-supported SOFC– Equations for mass-, heat- and momentum transport and internal reforming

reactions are solved simultaneously

• The surface area ratio is varied to study the effect on:– Reforming reaction rate– Mole fraction distribution

• An increased surface area ratio makes the conversion of CH4 to H2 and CO faster

• The maximum molar ratio of CO and H2 is increased

• Change of pressure or temperature have similar effects as the SA.


Future work


Future work

• Microscale modeling– Couplings of multiscale phenomena

• Ionic transport in the electrolyte• New fuel mixtures• Reforming reaction rates, depending on catalytic surface kinetics • Experimental work for model validation


Future work / Multi-step reaction scheme

CH4

H2

H2O

CO2

CO

CH4(s)

CH3(s)

CH2(s)

CH(s)

C(s)

HCO(s) CO (s)

CO2(s)

H2O(s)H(s)

H(s)

H(s)

H2


Do you want to learn more?

• November 27th (13:15) – Numerical Heat Transfer,“COMSOLand Fuel Cells”

• December 8th (8:15) – Fuel Cell Technology, ”Fuel Cell Demonstration & Commercialization”

• Hedvig’s Master Thesis presentation, “CFD Simulations of Transport Processes including Chemical Reactions in SOFCs ”

SOFC Modeling Considering Internal Reforming by a Global Kinetics Approach��and My Research in GeneralArticles written AgendaIntroduction to fuel cellsIntroduction to fuel cellsSOFCsSOFCsSOFC Modeling Considering Internal Reforming by a Global Kinetics Approach��M. Andersson, J. Yuan & B. SundénIntroductionGlobal SOFC reactionsInternal reforming reactions - Pre-reformer vs. int. ref.Internal reforming reactions - reaction mechanismsInternal reforming reactions - reaction mechanismsMathematical modelMathematical model - geometryMathematical model / COMSOL Multiphysics (FEM)Mathematical modelMathematical modelMathematical modelMathematical model - AssumptionsInternal reforming reactions - Mathematical modelParameter study - surface area ratioResult - Steam reforming reaction rate Result - Water-gas shift reaction rateResult - Methane mole fractionResult - Carbon monoxide mole fractionResult - Water mole fractionResult - Hydrogen mole fractionConclusionsFuture workFuture workFuture work / Multi-step reaction schemeDo you want to learn more?

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SOFC Modeling Considering Internal Reforming by …...SOFC Modeling Considering Internal Reforming by a Global Kinetics Approach and My Research in General Martin Andersson Division