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Operational behavior and reforming kineticsover Ni/YSZ of a planar type pre-reformer forSOFC systems

2nd World Congress on

Petrochemistry and Chemical EngineeringOctober 29th, 2014 Las Vegas, USA

SOFC systems

Van Nhu Nguyen, Ludger Blum, Roland Peters

Forschungszentrum Jülich GmbH, Germany

va.nguyen@fz-juelich.de

• Introduction

• Global reaction kinetics

• Experimental setup

• Results

Outline

Institute of Energy and Climate Research – Electrochemical Process Engineering (IEK-3) 2

• Modeling

• Conclusions

Basic layout of the SOFC system

Reaction in the Cathode:

Institute of Energy and Climate Research – Electrochemical Process Engineering (IEK-3) 3

O2 + 4 e- ↔ 2 O2-

Reactions in the Anode:

H2 + O2- ↔ H2O + 2 e-

CO + O2- ↔ CO2 + 2 e-

H2, CO, CH4, CO2, H2O

Steam reforming reactions of methane

Reaction -∆H (25 °C) [kJ/mol]

R1

R2

R3

CH4 + H2O ↔ CO + 3 H2

CH4 + 2 H2O ↔ CO2 + 4 H2

CH4 + CO2 ↔ 2 CO + 2 H2

-206

-165

-247

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R4

R5

R6

R7

CO + H2O ↔ CO2 + H2

CH4 ↔ C + 2 H2

2 CO ↔ C + CO2

CO + H2 ↔ C + H2O

41

-75

172

131

Models for global reaction kinetics

⋅

−⋅⋅⋅=

TR

EexpppFr aβ

OH

α

CHArrr, 24

( )2

OHOHCHCH

OHCHOHCH

Langr,

2244

2424

KpKp1

KKppkr

⋅++

⋅⋅⋅=

1) Arrhenius type

2) Langmuir-Hinshlewood type

Institute of Energy and Climate Research – Electrochemical Process Engineering (IEK-3) 5

⋅⋅

⋅−⋅⋅⋅=

OHCHSTRe,

3

HCO

OHCHeqr,

24

2

24 ppK

pp1ppkr

⋅⋅

⋅−⋅⋅=

OHCOse,

H2CO

COss

2

2

ppK

pp1pkr

3) Equilibrium approach

4) Water shift reaction approach

β

OH

α

CHArrr, 24ppkr ⋅⋅=

Global reaction kinetics (Arrhenius type)

Literature data review: inconsistent results of kinetics *

�: 0.85 – 1.4 (����=1 was used very often)

�

* Andersson M, Yuan J, Sunden B,

Applied Energy 2010; 87:1461

⋅

−⋅=

TR

EexpFk a

where

Institute of Energy and Climate Research – Electrochemical Process Engineering (IEK-3) 6

Motivation:

• Find the ”real” global kinetics of steam reforming reactions

• Optimization of pre-reformer for SOFC-system

� ����

�: negative and positive values

Ea : 42 - 208 kJ/mol

Design of a 5-layer pre-reformer using air heater

Catalyst: Ni/YSZ (Ni + 8 mol% Y2O3-stabilized ZrO2)

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Catalyst

s

Flow scheme of the experimental setup

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The comparison between the compositions for equilibrium and

measurement as function of temperature and space-time

• dew-point-measurement

• gas-chromatographic method

Analytical methods

Results

ratefeedvolumetric

volumereactor

v

V

o

==τ

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y = e-1,861x

0.6

0.8

1

/ x

CH

4,i

nle

t

610°C

1-layer-reformer, S/C=2

Graphs of concentration versus space time

S/C=2, k = 46

70% AOGR, k = 155

80% AOGR, k = 372

The experimental data fit not first order kinetics for methane concentration

Institute of Energy and Climate Research – Electrochemical Process Engineering (IEK-3) 10

y = e-1,861x

y = e-2.25x

y = e-3.09x

0

0.2

0.4

0 0.2 0.4 0.6 0.8 1

xC

H4

/ x

root (τ/s)

1-layer-reformer, S/C=2

5-layer-reformer, S/C=2

70% Recycle

80% Recycle

ratefeedvolumetric

volumereactor

v

V

o

==τ

k = 6,3

k = 14

k = 35

k = 66

k = 106

0

5

10

15

20

25

30

35

40

0.000 0.100 0.200 0.300 0.400

Inte

gra

ted

re

act

ion

ra

te

460°C

500°C

580°C

660°C

740°C

1-layer-reformer

Integrated reaction ratesfrom exp. data of the both reformers

k = 3518

20

ratefeedvolumetric

volumereactor

v

V

o

==τ

Modeling (Arrhenius type)

1- and 5-layer-reformer

�� ��

= 2; ���� =1

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0.000 0.100 0.200 0.300 0.400

Space time / s

( )

( ) ( )∫ ⋅=−⋅−

τ

0 OH0,

2

CH0,

τkξxξx

ξd

24

ξ: Progress variable of

reforming reaction

k = 13

k = 35

0

2

4

6

8

10

12

14

16

18

0 0.1 0.2 0.3 0.4 0.5

Inte

gra

ted

re

act

ion

ra

te

Space time /s

500°C 1-layer

500°C 5-layer

580°C 1-layer

580°C 5-layer

�� ��

= 2; ���� =1

ln(k) = -6,314 * (1000/T) + 10,87

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

1.1 1.2 1.3 1.4 1.5 1.6

ln(k

)

S/C=2,0

S/C=2,5

S/C=3,0

5-layer-reformer

Ea = 53 kJ/mol*

1-layer-reformer

Temperature: 460°C – 740°C

Temperature: 350°C – 620°C

* Nguyen,V.N., Blum, L., Peters, Ro.,Int. J. Hydrogen Energy (2014) 39: 7131

Temperature dependency of reaction rate

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1.1 1.2 1.3 1.4 1.5 1.6

(1/T) * 1000

ln(k) = -6,524 *(1000/T) + 11,316

0

1

2

3

4

5

6

0.9 1 1.1 1.2 1.3 1.4

ln(k

)

(1/T) *1000

1-layer-reformer

S/C=2.0

S/C=2.5

S/C=3.0

Ea = 54 kJ/molEa = 50 kJ/mol**

** Drescher I. Kinetik der Methan-Dampf-Reformierung. Diss RWTH Aachen 1999.

Ea = 38.5 - 62 kJ/mol****** Liu, K., Song, C., Subramani, V. (Eds.)Wiley & Sons Publication; 2010.

k = 25

k = 46

k = 372

0

50

100

150

200

250

300

350

0.000 0.200 0.400 0.600 0.800 1.000

Inte

gra

ted

re

act

ion

ra

te

480°C

520°C

610°C

80% AOGR

AOGR at 70% fuel utilization

Example at 610°C:

Without AOGR, k = 46

70% AOGR, k = 155

80% AOGR, k = 372

Ea = 117 kJ/mol

Effect of anode off-gas recycling (AOGR)

�� ��

= 2; ���� =1

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ln(k) = -14,09 * (1000/T) + 21,80

ln(k) = -14,15 * (1000/T) + 21,04

0

1

2

3

4

5

6

7

1.1 1.2 1.3 1.4

ln(k

)

1000/T

80% AOGR

70% AOGR

0.000 0.200 0.400 0.600 0.800 1.000

Space time / s

( )

( ) ( )∫ ⋅=−⋅−

τ

0 OH0,

2

CH0,

τkξxξx

ξd

24

Ea = 117 kJ/mol

ξ: Progress variable of

reforming reaction

• Two different planar pre-reformers containing Ni/YSZ catalyst were tested for

operational behavior and kinetics of methane steam reforming reactions in a

temperature-range of 350°C - 740°C.

• Experimental results for the two reformers are close to each other.

• The developed kinetic expression of Arrhenius type (second order with respect

to mole fraction of methane and first order with respect to mole fraction of water)

gives a good agreement with the experimental results.�� ��

����

Conclusions

Institute of Energy and Climate Research – Electrochemical Process Engineering (IEK-3) 14

gives a good agreement with the experimental results.

• This kinetic expression (

�� ��

= 2; ���� =1) is universally applicable for different steam

to carbon ratios and also for the case of anode off-gas recycling (AOGR).

• In the case of anode off-gas recycling the reaction rate constant is larger than

that without AOGR.

• Understanding of the methane steam reforming reactions is expected to be of

significant importance for the further development of SOFC systems.

Acknowledgement goes to allstaff members of JÜLICH for their excellent work

and to the Helmholtz Society for financing these activities

Institute of Energy and Climate Research – Electrochemical Process Engineering (IEK-3) 15

Thank you for your attention!