BRE Luncheon Workshop, 29 April 2010

2D CFD Modeling of ammonia fueled solid oxide fuel cells with

proton conducting electrolyte *

Meng Ni

Department of Building and Real Estate

The Hong Kong Polytechnic University

ABSTRACT

The use of ammonia as a fuel for solid oxide fuel cells (SOFCs) has received increased

attention for clean power generation. In this study, a 2D model is built to investigate the coupled

transport and chemical/electrochemical reactions in a planar SOFC with proton conducting

electrolyte (SOFC-H). The model consists of an electrochemical model relating the current

density-voltage characteristics and a 2D computational fluid dynamics (CFD) model simulating

the heat and mass transfer phenomena. The conservation laws for mass, momentum, energy and

species are discretized and solved with the finite volume method (FVM). The coupling of

pressure and velocity is treated with SIMPLEC algorithm. Simulations are conducted to study

the distributions of current density, electrolyte Nernst potential, rate of ammonia thermal

cracking and gas composition in the SOFC. The effects of operating potential and temperature

on the rate of chemical/electrochemical reactions and the electric output of ammonia fueled

SOFC-H are examined.

Keywords:

Solid oxide fuel cells; computational fluid dynamics; heat transfer; ammonia thermal cracking

* Paper accepted for presentation at FUELCELL2010, June 2010, New York, USA

http://www.asmeconferences.org/FuelCell2010/

2D CFD Modeling of NH3 Fueled SOFCs With Proton

Conducting Electrolyte

Meng Ni

Department of Building and Real Estate

The Hong Kong Polytechnic University

Hong Kong, China

Email: bsmengni@inet.polyu.edu.hk

Tel: 852 – 2766 4152

Building and Real Estate Workshop, 29 April 2010

Full paper accepted for oral presentation at Fuelcell 2010

http://www.asmeconferences.org/FuelCell2010/

Copyright @ 2010 by ASME

Contents

1. Background

2. Model development

3. Results and analysis

4. Conclusions

2

Background – what are fuel cells?

3

Fuel Cells: can convert chemical energy of a fuel to

electricity efficiently, cleanly and quietly; much more

advanced than conventional combustion engines.

Suitable fuels for fuel cells are hydrogen (produced from

renewable sources) and biomass-derived renewable bio-gas.

Types of fuel cells

4

Presently too expensive; efficiency need to be further improved

Low temperature fuel cells (typically below 373K):

� Alkaline fuel cell (AFC)

� Phosphoric acid fuel cell (PAFC, can be >373K)

� Direct alcohol fuel cell (DAFC)

� Direct methanol fuel cell (DMFC)

� Proton exchange membrane fuel cell (PEMFC)

Applications: vehicles, portable mobile power sources

Toshiba notebook

Photos taken by Meng Ni

Types of fuel cells

5

High temperature fuel cells (673 K – 1273 K):

� Molten carbonate fuel cell (MCFC)

� Solid oxide fuel cell (SOFC)

Applications: vehicles, stationary power sources

Co-generation

Siemens Westinghouse's 250 KW

combined heat and power fuel cell system

Key developers:General Electric (USA)

Siemens Westinghouse (USA)

Rolls-Royce (Europe)

Sulzer Hexis (Europe)

Mitsubishi Heavy Industries (Japan)

Ceramic Fuel Cells (Australia)

… …

Advantages of working at a high temperature

6

1. High electrode reactivity – use of cheap catalyst (Ni)

2. Fast ion conduction – minimal ohmic overpotential

3. Feasibility of direct internal reforming – fuel flexibility

(bio-ethanol, bio-methanol, natural gas, ammonia can

be used as fuels)

4. High quality waste heat recoverable to achieve high

efficiency (cogeneration for building applications)

Challenges:

1. Long-term stability

2. Slow start-up

3 ……

Ammonia fed SOFCs

Problems of hydrogen fuel – production and storage

Use of alternative fuels in SOFCs – biofuels, ammonia

Ammonia is a by-product in chemical industry, i.e. as a fertilizer for

agricultural crops; the infrastructure of ammonia is mature; the

production, transportation and storage of ammonia is mature; can

be used as an energy storage media; It could be manufactured

from renewable energy sources, as well as coal or nuclear power.

7

Experimental studies in literature•A. Wojcik, H. Middleton, I. Damopoulos, J. Van herle, Ammonia as a fuel in solid oxide fuel cells, J. Power Sources

118 (2003) 342-348.

•Q.L. Ma, R.R. Peng, Y.J. Lin, J.F. Gao, G.Y. Meng, A high performance ammonia fed solid oxide fuel cell, J. Power

Sources 161 (2006) 95-98.

•Q.L. Ma, R.R. Peng, L.Z. Tian, G.Y. Meng, Direct utilization of ammonia intermediate temperature solid oxide fuel

cells, Electrochem. Commun. 8 (2006) 1791-1795.

•G.G.M. Fournier, I.W. Cumming, K. Hellgardt, High performance direct ammonia solid oxide fuel cell, J. Power

Sources 162 (2006) 198-206.

•Q.L. Ma, J.J. Ma, S. Zhou, R.Q. Yan, J.F. Gao, G.Y. Meng, A high-performance ammonia fueled SOFC based on a

YSZ thin-film electrolyte, J. Power Sources 164 (2007) 86-89.

•N.J.J. Dekker, G. Rietveld, J. Fuel Cell Sci. Technol. 3 (2006) 499-502.

•N. Maffei, L. Pelletier, J.P. Charland, A. McFarlan, An ammonia fuel cell using a mixed ionic and electronic

conducting electrolyte, J. Power Sources 162 (2006) 165-167.

•L. Pelletier, A. McFarlan, N. Maffei, Ammonia fuel cell using doped barium cerate proton conducting solid

electrolytes, J. Power Sources 145 (2005) 262-265.

•N. Maffei, L. Pelletier, J.P. Charland, A. McFarlan, An intermediate temperature direct ammonia fuel cell using a

proton conducting electrolyte, J. Power Sources 140 (2005) 264-267.

•N. Maffei, L. Pelletier, J.P. Charland, A. McFarlan, A direct ammonia fuel cell using barium cerate proton

conducting electrolyte doped with Gadolinium and Praseodynium, Fuel Cells 7 (2007) 323-328.

•N. Maffei, L. Pelletier, A. McFarlan, A high performance direct ammonia fuel cell using a mixed ionic and

electronic conducting anode, J. Power Sources 175 (2008) 221-225.

•L. Zhang, Y. Cong, W. Yang, L. Lin, A direct ammonia tubular solid oxide fuel cell, Chin. J. Catal. 28 (2007) 749-

751.

•G.Y. Meng, C.R. Jiang, J.J. Ma, Q.L. Ma, X.Q. Liu, Comparative study on the performance of a SDC-based SOFC

fueled by ammonia and hydrogen, J. Power Sources 173 (2007) 189-193.

•L. Zhang, W. Yang, Direct ammonia solid oxide fuel cell based on thin proton conducting electrolyte, J. Power

Sources 179 (2008) 92-95.

Model development

9

1D view of SOFC fed with ammonia

Energy losses: 1. Activation overpotential (resistance to electrochemical reaction)

2. Concentration overpotential (resistance to gas transport)

3. Ohmic overpotential (resistance to electron/ion conduction)

The fluid flow, heat transfer, chemical reaction, electrochemical reaction are all considered.

10

2D Thermo-electrochemical Model

Anode

CathodeElectrolyte

Interconnect

Interconnect

NH3H2

H2, N2; NH3

Air (O2; N2) O2; N2; H2OH2OO2

x, U

y, V y=0

y = yL

x = 0 x = xL

A

B C

D

NH3 H2+N2

2D model

11

Electrochemical model – developed in 1D model

Computational fluid dynamic model

, ,act a act c ohmV E η η η= − − −

( )

+=

Int

cOH

Int

cO

Int

aH

P

PP

F

RTEE

,

5.0

,,

0

2

22ln2

, i = a, c( ), ,

0,

1exp exp

act i act i

i

zF zFJ J

RT RT

α η α η − = − −

3

0.196200exp100.4 15

NHPRT

r

−×=

ohmic IonicJLRη =

Key equations for the electrochemical model

Governing equations - continued

12

Key equations of the CFD model are summarized below:

( ) ( )m

U VS

x y

ρ ρ∂ ∂+ =

∂ ∂

( ) ( )x

UU VU P U US

x y x x x y y

ρ ρµ µ

∂ ∂ ∂ ∂ ∂ ∂ ∂ + = − + + +

∂ ∂ ∂ ∂ ∂ ∂ ∂

( ) ( )y

UV VV P V VS

x y y x x y y

ρ ρµ µ

∂ ∂ ∂ ∂ ∂ ∂ ∂ + = − + + +

∂ ∂ ∂ ∂ ∂ ∂ ∂

( ) ( )P P

T

c UT c VT T Tk k S

x y x x y y

ρ ρ∂ ∂ ∂ ∂ ∂ ∂ + = + +

∂ ∂ ∂ ∂ ∂ ∂

( ) ( ), ,i m i m

i i eff effi isp

UY VY Y YD D S

x y x x y y

ρ ρρ ρ

∂ ∂ ∂ ∂∂ ∂ + = + +

∂ ∂ ∂ ∂ ∂ ∂

( )1f sk k kε ε= + −

( ), ,1p p f p s

c c cε ε= + −

Continuity

Momentum

Energy

Species

The electrochemical model is incorporated into CFD

model through source terms

Model development - continued

Boundary conditions….

Numerical method – finite volume method

SIMPLEC algorithm

TDMA based Iteration

Program written in Fortran

… …

13

Results –

14

As the paper has been accepted for oral presentation at FuelCell 2010 and full

paper technical publication in the ASME proceedings, Copyright has been

transferred to ASME.

Figures can not be made electronically available. They will be presented at the

workshop.

Sorry for any inconvenience caused.

Conclusion

15

• A 2D numerical model is developed to study the performance of direct NH3-fueled SOFC-H, by

integrating the previously developed electrochemical model with a 2D CFD model.

• The direct use of NH3 fuel in SOFC-H significantly influence the distributions of gas composition,

temperature field as well as the electrochemical performance.

• The inlet temperature significantly influences the SOFC-H. At a higher inlet temperature of 973K,

NH3 thermal cracking rate near the inlet is in the order of 104 mol.m-3.s-1 but decreases sharply in the

downstream, due to a fast decrease in SOFC-H temperature along the flow channel. At an inlet

temperature of 773K, the NH3 thermal cracking rate is in the order of 100 and does not vary much along

the main flow stream.

• Although a high working temperature favors high electrochemical performance of NH3-fueled

SOFC-H, the large temperature gradient and the thermal stress in the solid structure must be considered

with caution.