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

Transcription

1 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

2 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 FuelCell , 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

3 Agenda Introduction to FCs and SOFCs ECS/SOFC XI article Introduction Literature survey Mathematical model Results Conclusions Future work

4 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

5 Introduction to fuel cells Willingness To Pay (WTP) for different FC niche markets

6

7 SOFCs Working temperature: 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

8 SOFCs

9 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, Vienna October 9th, 2009

10 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.

11 Global SOFC reactions

12 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

13 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

14 Internal reforming reactions - reaction mechanisms Steam reforming reaction m varies between 0.85 and 1.4 n varies between and 1 E a varies between 60 and 230 kj/mol Water gas shift reaction Normally assumed to be in equilibrium = T E p p k r a n O H m CH r R exp 2 4 = O H CH r e H CO O H CH eq r p p K p p p p k r , 3, 1 = O H CO s e H CO CO s s p p K p p p k r 2 2 2, 1

15 Mathematical model

16 Mathematical model - geometry

17 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

18 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

19 Mathematical model Fuel utilization: 80 % Oxygen utilization: 20 % Current density: 0.3 A/cm 2 Inlet temperature: 1100 K 30 % pre reformed natural gas

20 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

21 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

22 Internal reforming reactions - Mathematical model Steam reforming depends on active surface area Parameter study is preformed r = SA kr, f p CH 4 p H 2 O exp Ea, r, f RT 3 kr, r p CO p H 2 exp Ea, r, r RT k = f (T) Water-gas shift reforming In fuel channel and anode r s = Ea, s, f Ea, ks, f pco ph O exp ks, r pco ph exp RT RT s, r [Klein et al, Chem. Eng. Sci. 62, (2007)]

23 Parameter study - surface area ratio SA a = m 2 /m 3 SA b = *10 m 2 /m 3

24 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.

25 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 electrochemical reactions.

26 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 reaction in the porous structure.

27 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 reaction in the porous structure.

28 Result - Water mole fraction The highest fraction (water) is found at the outlet. A high surface area ratio means that water is consumed faster (reforming reactions), than generated (electrochemical reactions).

29 Result - Hydrogen mole fraction A high surface area ratio means that hydrogen is generated faster (reforming reactions), than consumed (electrochemical reactions).

30 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 CH 4 to H 2 and CO faster The maximum molar ratio of CO and H 2 is increased Change of pressure or temperature have similar effects as the SA.

31 Future work

32 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

33 Future work / Multi-step reaction scheme CH 4 H 2 O CH 4 (s) H(s) CH 3 (s) H 2 O(s) CO H 2 CH 2 (s) HCO(s) CO (s) H(s) CH(s) H(s) CO 2 (s) C(s) H 2 CO 2

34 Do you want to learn more? November 27th (13:15) Numerical Heat Transfer, COMSOL and 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