of Multi-Fuels in a Micro Channel

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Effect of Catalyst Segmentation with cavities on Combustion Enhancement

of Multi-Fuels in a Micro Channel

Yueh-Heng Li¹, Guan-Bang Chen², Fang-Hsien Wu¹, Tsarng-Sheng Cheng³, Yei-Chin Chao¹

Speaker：Fang-Hsien Wu

1Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan, Taiwan, ROC

2Energy Technology and Strategy Research Center, National Cheng Kung University, Tainan, Taiwan ROC

3Department of Mechanical Engineering, Chung Hua University, Hsinchu, 300, Taiwan, ROC

ICDERS-23^rd University of California, Irvine, USA 2011/07/24 ~ 2011/07/29

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Introduction



In the miniaturizing process of a reactor size

 Increasing surface to volume ratio (S/V)

• increased heat loss to the wall

• the possibility of radical termination by wall reactions.

 Characteristic dimension < 1 mm

• enhanced heat loss to the wall

• homogeneous reaction are quenched



How to overcome the shortcomings

 Use quench-resistant fuel(Norton et al. 2005), thermal recuperating concept(Sitzki et al. 2001) or catalyst surface(Chao et al. 2004) to solve this weakness.

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Catalyst combustion



Catalytic combustion involves the coupling of the process of reactive flow (homogeneous reaction) and the process on the catalyst (heterogeneous reaction).



Hetero-/homogeneous interaction

 Promotion of gas-phase reaction due to the catalytically induced exothermicity.

 Inhibition of gaseous reaction due to near wall catalytic fuel depletion.

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Catalytic combustion of methane



Hetero-homogeneous coupling in methane combustion

over Pt tube :

(Westbrook and Dryer, 1981; Norton and Vlachos, 2003)

 Gas-phase combustion of methane can be described by

• Incomplete oxidation of CH₄ to CO

• Main heat-releasing oxidation of CO to CO₂

Pt Gas reaction

CH₄→ CO

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Catalytic combustion of methane

Pt CO

Gas reaction



Hetero-homogeneous coupling in methane combustion

over Pt tube :

 Gas-phase combustion of methane can be described by

 CO has active characteristics on the Pt due to its high sticking coefficient (0.85 on Pt)

 By depriving CO for the gas-phase, the catalyst inhibits the

onset of homogeneous ignition.

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Catalytic combustion of methane

Gas reaction surface reaction

Pt

CO ＋ O₂ CO₂ CO₂

CO+OH



Hetero-homogeneous coupling in methane combustion

over Pt tube :

 Gas-phase combustion of methane can be described by

 CO has active characteristics on the Pt due to its high sticking coefficient (0.85 on Pt)

 By depriving CO for the gas-phase, the catalyst inhibits the onset of homogeneous ignition.

 Intermediate species( H, OH) enhance CO gas reaction.

(CO+OH→CO

₂

+H >> CO+O

₂

→CO

₂

)

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Catalytic combustion of methane

Gas reaction surface reaction

Competition of hetero- and homogeneous interaction

(Surface reaction) (Gas reaction)



Hetero-homogeneous coupling in methane combustion

over Pt tube :

 Gas-phase combustion of methane can be described by

 CO has active characteristics on the Pt due to its high sticking coefficient (0.85 on Pt)

 By depriving CO for the gas-phase, the catalyst inhibits the onset of homogeneous ignition.

 Intermediate species( H, OH) enhance CO gas reaction.

(CO+OH→CO

₂

+H >> CO+O

₂

→CO

₂

)

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Motivation and Object

ISSUE 1

 To modify the correlation of hetero-/homogeneous interaction of hydrocarbon fuel over platinum in a wider stable operation range.

 Segment catalyst － providing sufficient chemical radical and catalytically induced exothermicity.

(Chen et al., 2008)

Cooperation between hetero- and homogeneous reaction

 Cavity － to provide a low-velocity field to stabilize the gas reaction.

(Srivatsava et al., 2009)

ISSUE 2

 To identify the interplay between H₂/CO, CH₄/CO, CH₄/H₂ in a micro reactor.

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Numerical model and chemical mechanism

 h = 1mm, L = 30mm , w = 1mm, d = 0.2mm

 CFD-ACE +, Multi-step reaction mech. (GRI-3.0)

 Non-uniform grids were used with more grids distributed in the reaction region to provide sufficient grid resolution.

 Laminar flow (ER = 1.0, T_in= 300K)

h/2

L

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 Three catalyst layouts of a micro reactor were studied:

 Single catalyst, (10mm Pt length)

 Segment catalyst with inert wall, (2mm Pt sec. ×5 )

 Segment catalyst with cavities. (2mm Pt sec. ×5 )

ISSUE 1

wall Pt

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Y(mm) -0.9

0

0.9 ^CH4: 0.0000 0.0113 0.0225 0.0338 OH: 0.0E+00 2.0E-03 4.0E-03 6.0E-03

(a)

Y(mm)

-0.9 0 0.9 (b)

Axial distance (cm)

Y(mm)

0 0.5 1 1.5 2 2.5 3

-0.9 0 0.9 (c)

Results and discussion

 Homogeneous combustion cannot be sustained in a micro- reactor with non-catalytic walls under this condition.

 In view of multi-segment and cavity, methane attains complete consumption in a short distance and flame anchoring moves

upstream in high velocity

condition. Flow velocity: 10 m/sec Equivalence ratio: 0.6

Single catalyst

Segment catalyst

Segment catalyst with cavities

 Methane combustion characteristics for various catalyst layouts

CH₄

OH CH₄

OH OH

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14.0

2.0 10.0

18.0 22.0 26.0 34.0

38.0 2.0

0.002

0.002 0.026

0.034 0.008

Y(mm)

0 0.2 0.4 0.6 0.8 1

-0.9 0

0.9 OH

8.0E-03

1.0E-04 CH4 0.0338

0.0000 CO

Velocity Magnitude

Results and discussion

 Onset of heterogeneous reaction occurs in first two catalyst segmentation.

 The space between catalyst

segmentation sustains gas reaction.

 CO is formed behind the gas reaction, and is consumed on the following catalyst surface.

Catal. Segmentation Catal. Segmentation and Cavity

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14.0

2.0 10.0

18.0 22.0 26.0 34.0

38.0 2.0

0.002

0.002 0.026

0.034 0.008

Y(mm)

0 0.2 0.4 0.6 0.8 1

-0.9 0

0.9 OH

8.0E-03

1.0E-04 CH4 0.0338

0.0000 CO

Velocity Magnitude

Results and discussion

 Onset of heterogeneous reaction occurs in first two catalyst segmentation.

 The space between catalyst

segmentation sustains gas reaction.

 CO is formed behind the gas reaction, and is consumed on the following catalyst surface.

Catal. Segmentation Catal. Segmentation and Cavity

6.0 2.0

14.0

14.0 24.0

22.0 26.0

18.0 30.0 34.0

2.0

22.0

40.0 30.0 38.0

0.008 0.014

0.002

0.030

0.018

0.034

0.028 0.020

0.016

Y(mm)

0 0.2 0.4 0.6 0.8 1

-0.9 0

0.9 _OH

6.0E-03

0.0E+00

CH4 0.0338

0 Velocity Magnitude

CO

2.0 2.0

2.0

10.0 10.0

18.0 18.0

18.0

26.0

34.0 2.0

10.018.0 26.0

34.0

0.008 0.014

0.002

0.030

0.018

0.034

0.028 0.020

0.016

Y(mm)

0 0.2 0.4 0.6 0.8 1

-0.9 0

0.9 _OH

6.0E-03

0.0E+00

CH4 0.0338

CO

2.0 8.0

14.0

18.0

20.0 32.0

38.0 2.0 12.0 28.0

40.0

0.008 0.014

0.002

0.030

0.018

0.034

0.028 0.020

0.016

Y(mm)

0 0.2 0.4 0.6 0.8 1

-0.9 0

0.9 _OH

6.0E-03

0.0E+00

CH4 0.0338

CO

 Cavity provides a low-velocity zone to stabilize the gas reaction.

 Congregation of OH radicals in the cavities presents flame anchoring.

 Flame anchoring location is prone to move upstream compared to catalyst segmentation case.

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ISSUE 2

 Three binary-fuel compositions were studied:

 50%H

₂

+50%CO

 50%CH

₄

+50%CO

 50%CH

₄

+50%H

₂

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1.5E-03 5.0E-04

3.0E-03

2.0E-03

4.0E-03 3.0E-03 2.5E-03

2.5E-03 3.0E-03

3.5E-03

4.5E-03

0.02 0.1

0.04 0.16

0.2 0.12 0.22

0.14

(mYm) 0.18

0 0.5 1 1.5 2 2.5 3

-0.9 0

0.9 _H2

0.012 0.009 0.006 0.003 0.000 CO 0.162 0.108 0.054 0.000 OH

CO2

5.0E-04 1.0E-03

1.0E-03

4.0E-03 2.5E-03 3.0E-03

4.0E-03

2.0E-03 3.0E-03

2.0E-03 5.0E-03

1.5E-03

2.5E-03 3.5E-03

0.06 0.1 0.18

0.12 0.14

0.16

0.2 0.02

(mYm) 0.22

0 0.5 1 1.5 2 2.5 3

-0.9 0 0.9

CO 0.162 0.108 0.054 0.000 H2 0.012 0.009 0.006 0.003 0.000 OH

CO2

4.5E-03 1.0E-03

2.0E-03 5.0E-04

4.0E-03

4.0E-03 5.0E-03 2.5E-03

1.5E-03 2.0E-03

3.0E-03

0.02 0.020.06

0.14 0.18 0.2 0.22

Y(mm)

0 0.5 1 1.5 2 2.5 3

-0.9 0 0.9

CO 0.162 0.108 0.054 0.000 H2 0.012 0.009 0.006 0.003 0.000 OH

CO2

Results and discussion

 H₂ and CO have high sticking coefficients to Pt.

 H₂ has large mass diffusivity, so H₂ reaches the catalyst bed and triggers the heterogeneous reaction.

 No significant difference between the two catalyst configurations.

Flow velocity: 10 m/sec Equivalence ratio: 1.0



Binary-fuel combustion feature – H

₂

and CO

Single catalyst

Segment catalyst

Segment catalyst with cavities

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Results and discussion

H₂ attains complete conversion prior to CO conversion.

 H₂ tends to lightoff early on the surface, providing catalytically induced exothermicity and a lot of chemical radicals(H, OH) to support the hetero-/homogeneous reaction of CO.

 Onset of gas reaction recedes downstream with an increase of flow velocity.



Binary-fuel combustion feature – H

₂

and CO

H2massfraction COmassfraction

0 0.5 1 1.5 2 2.5 3

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

0 0.05 0.1 0.15 0.2

H2 (10m/s) H2 (20m/s) H2 (30m/s) CO (10m/s) CO (20m/s) CO (30m/s)

OHmassfraction

0 0.5 1 1.5 2 2.5 3

0 0.001 0.002 0.003 0.004 0.005 0.006

10m/s 20m/s 30m/s

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Results and discussion

 CO has a high sticking coefficient to platiunm.

 CH₄ become depleted in the first two catal. segments. Incomplete combustion yields a high

concentration of CO species.

 The following segment catalyst helps deplete CO in the residual

mixture until a complete conversion.

 In the case with segment catal. and cavities, the gas reaction moves upstream and anchors in the first cavity.

Segment catalyst Segment catalyst with cavities



Binary-fuel combustion feature – CH

₄

and CO

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Results and discussion

Catalyst Inert wall

CH₄

CO High Sticking coefficient to platinum



Binary-fuel combustion feature – CH

₄

and CO

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 CO surface reaction dominates in the first segment catalyst, providing exothermicity to pre-react methane and convert to inert wall.

Catalyst Inert wall

CH₄

CO ＋ O² CO

surface reaction

Catalytically induced exothermicity

Results and discussion



Binary-fuel combustion feature – CH

₄

and CO

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Catalyst CH₄

CO O₂ CO

 CH

₄

releases H and OH radicals to assist CO conversion in the following gas reaction.

Inert wall

＋

Gas reaction

CO ＋ OH

H CO₂

Results and discussion



Binary-fuel combustion feature – CH

₄

and CO

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Catalyst Inert wall

＋

Results and discussion

 This phenomenon is a so-called cooperation between hetero- and homogeneous reaction.



Binary-fuel combustion feature – CH

₄

and CO

CH₄

CO O₂ CO

Gas reaction

CO ＋ OH

H CO₂

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 First 1 mm：

CH₄ depletes in the segmentation catalyst and yields CO and CH₃.

 In the following 1 mm：

the OH distribution in the cavity implies gas reaction existing so that CH₄ and CH₃ can further deplete in this region.

Results and discussion

Axial distance (mm)

Massfraction

0 0.5 1 1.5 2

0 0.02 0.04 0.06 0.08 0.1 0.12

CH4-seqmented+cavity CO-seqmented+cavity CH4-seqmented CO-segmented

Massfraction

0 0.5 1 1.5 2

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007

CH3-S+C OH-S+C CH3-S OH-S



Binary-fuel combustion feature – CH

₄

and CO

Catalyst length Catalyst length

Cavity Cavity

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Results and discussion

 For high flow velocities, catalytically induced

exothermicity from H₂ in the single and multi-segment catalyst cases is unable to ignite the gas reaction of methane due to heat loss.

 Cavity plays an important role to decelerate flow velocity in localized space and collect radicals from up stream.



Binary-fuel combustion feature – CH

₄

and H

₂

Catalyst Inert wall

CH₄

H₂ O₂

Heat loss No gas reaction

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 Temperature leap and OH congregation imply gas reaction in the cavity.

 H₂ is reduced first in the catalyst section, and abruptly rises by dissociation of methane in the cavity section.

Results and discussion



Binary-fuel combustion feature – CH

₄

and H

₂

Temperature(K) Massfraction

0 0.5 1 1.5 2

1300 1400 1500 1600 1700 1800

0 0.01 0.02 0.03 0.04 0.05

Temperature CH4 H2(x10)

Massfraction

0 0.5 1 1.5 2

-0.01 0 0.01 0.02 0.03 0.04 0.05 0.06

CO CH3 OH

Catalyst length

Catalyst length Cavity Cavity

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ISSUE 1

 Heterogeneous reaction in the prior catalyst segment can produce active chemical radicals and catalytically induced exothermicity.

 Homogeneous reaction is subsequently induced and anchored in the following cavity.

 Cavity can collect radicals and maintain wall temperature, so that it can successfully sustain gas reaction in a high flow velocity even though H₂ provides low volumetric energy density.

 These processes of multi-fuel catalytic combustion belong to mutual assisting coupling between the heterogeneous and homogeneous reaction, instead of mutual competing in a conventional catalyst reactor.

Conclusions

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ISSUE 2

 H₂/CO mixture can sustain in high flow velocity in two catalyst configurations caused by their high sticking coefficients, so that CO/H₂ can lightoff on catalyst segment.

 CH₄ /CO mixture can be stabilized in high flow velocity in two catalyst configurations.

 Upstream catalyst segment incomplete combustion of CH₄ yields CO.

 Following catalyst segments fully consume CO due to the preferred CO catalytic reaction of high sticking coefficient.

 CH₄/H₂ mixture only multi-segment catalyst with cavity can stabilize gas reaction in a high flow velocity.

Conclusions

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Thank you For your attention !!

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Results and discussion

 For smaller flow velocity, CH₄ can deplete within a short

distance of 2 mm.

 OH concentration distribution moves downstream depending on the increasing inlet flow.

 This layouts not only extends the blowout limit of the system, but also controls the location of flame anchoring and heat release in the micro-reactor system.

Flow velocity: 5,7,11,15 m/sec Equivalence ratio: 0.6

 Methane combustion characteristics for various inlet velocities

Y(mm)

0

0.9 _0.0065^OH

0.0000 V=5m/s

Y(mm)

0

0.9 _0.0065^OH

0.0000 V=7m/s

Y(mm)

0

0.9 _0.0065^OH

0.0000

V=11m/s

Y(mm)

0 0.5 1 1.5 2 2.5 3

0

0.9 _0.0065^OH

0.0000

V=15m/s

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Results and discussion

 Within the 10mm-long

catalyst bed, H₂ is completely consumed, but CO has no

significant reaction.

 Heterogeneous reaction of CO, it belongs to kinetic-control region in the catalyst section.

 CO conversion dominantly counts on homogeneous

reaction, while H₂ conversion equally relies on hetero- and homogeneous reactions.



Binary-fuel combustion feature in segment catal.– H

₂

and CO

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Axial distance (cm) 0

10 20 30 40 50 60 70 80 90 100

Ys/Yb(%)

CO mass fraction H₂ mass fraction

Heterogeneous reaction

Homogeneous reaction

5%

95%

Catalyst section

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 High thermal conductivity can deliver thermal heat from catalyst section to non-catalyst section for stabilizing gas reaction and even heat up the

downstream catalyst

segments for accelerating sequential surface reaction.

Results and discussion



Multi-fuel combustion feature in segment catal. and cavities

– CH

₄

and H

₂

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Axial distance (cm) 0

10 20 30 40 50 60 70 80 90 100 110

Ys/Yb(%) Silicom (124 W/m/K)

Platinum ( 69.1W/m/K) Cordierite ( 3.3W/m/K)

H₂ CH₄ mass fraction