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Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

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Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities. Yueh-Heng Li 1 , Guan-Bang Chen 2 , Fang-Hsien Wu 1 , Tsarng-Sheng Cheng 3 , Yei-Chin Chao 1 Speaker : Fang-Hsien Wu. 1 Department of Aeronautics and Astronautics, - PowerPoint PPT Presentation
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Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities Yueh-Heng Li 1 , Guan-Bang Chen 2 , Fang- Hsien Wu 1 , Tsarng-Sheng Cheng 3 , Yei-Chin Chao 1 Speaker Fang-Hsien Wu 1 Department of Aeronautics and Astronautics, National Cheng Kung University Tainan, Taiwan, ROC 2 Energy Technology and Strategy Research Center, National Cheng Kung University Tainan, Taiwan ROC 3 Department of Mechanical Engineering, Chung Hua University Hsinchu, 300, Taiwan, ROC The 34 nd International Symposium on Combustion, Warsaw University of Technology, Poland 2012/07/29 ~ 2012/8/3
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Page 1: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Combustion Characteristics in a Small-Scale Reactor with Catalyst

Segmentation and Cavities

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

Speaker : Fang-Hsien Wu1Department of Aeronautics and Astronautics,

National Cheng Kung University Tainan, Taiwan, ROC2Energy Technology and Strategy Research Center,

National Cheng Kung University Tainan, Taiwan ROC3Department of Mechanical Engineering,

Chung Hua University Hsinchu, 300, Taiwan, ROC

The 34nd International Symposium on Combustion, Warsaw University of Technology, Poland 2012/07/29 ~ 2012/8/3

Page 2: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

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 Quench-resistant fuel => hydrogen (Norton et al. 2005, Yuasa, 2005)

Thermal recuperating concept => reduce heat loss (Sitzki et al. 2001, Ronny et al. 2002, Federici et al.

2009) Catalytic surface => reduce radical depletion

(Norton et al. 2004, Chao et al. 2004, Karagiannidis 2007)

Page 3: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Catalyst combustion

Catalytic combustion involves the coupling of homogeneous reaction and 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.

Page 4: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Motivation and Object In general, we attempt to extend the stable operation range of the

small reactor, especially in high or low inflow velocity.

ISSUE Use a novel catalyst bed design of catalyst segmentation with

cavities to extend the operation range and to study H2 fuels reactions

in a catalytic micro-reactor. Catalyst segmentation - providing sufficient chemical radical and

catalytically induced exothermicity.

Catalyst segmentation with Cavity - to provide a low-velocity field to stabilize the gas reaction.

Page 5: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Experimental apparatus and method Reactor : 82 mm in length 20 mm in width 5 mm in height

Aluminum oxide ceramic stick : 60 mm (l) × 3 mm (w) × 2 mm (h)

Catalyst layouts : 8 segments (each with 2 mm catalyst) 4 segments (each with 4 mm catalyst) 2 segments (each with 8 mm catalyst) 16 mm catalyst without segmentation

Cavity : 2 mm (l) × 3 mm (w) × 1 mm (d)

600K

Page 6: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Experimental apparatus and method Reactor : 82 mm in length 20 mm in width 5 mm in height

Aluminum oxide ceramic stick : 60 mm (l) × 3 mm (w) × 2 mm (h)

Catalyst layouts : 8 segments (each with 2 mm catalyst) 4 segments (each with 4 mm catalyst) 2 segments (each with 8 mm catalyst) 16 mm catalyst without segmentation

600K

Page 7: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Experimental apparatus and method Reactor : 82 mm in length 20 mm in width 5 mm in height

Aluminum oxide ceramic stick : 60 mm (l) × 3 mm (w) × 2 mm (h)

Catalyst layouts : 8 segments (each with 2 mm catalyst) 4 segments (each with 4 mm catalyst) 2 segments (each with 8 mm catalyst) 16 mm catalyst without segmentation

600K

Catalyst segmentation with cavity 8 segments (each with 2 mm catalyst) 7 cavities : 2 mm (l) × 3 mm (w) × 1 mm (d)

Page 8: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Numerical model and chemical mechanism

Reactor : h = 1mm, L = 65mm , Cavity : w = 2mm, d = 1mm Total catalyst length = 16mm Laminar flow (Tin = 300K) CFD-ACE +, Homogeneous reaction mech. (Miller and Bowman),

Heterogeneous reaction mech. (Deutschmann et al.)

Non-uniform grids.

OutletPlatinum

Quartzs

Al2O3Cavity

UniformInlet

(L) 65 mm

1 mm

2 mm

2 mm

Tw,up (x)

Tw,down (x)

(h) 1 mm

w

d

Page 9: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Three catalyst layouts of a small-scale reactor were studied: Single catalyst, (16mm Pt length)Catalyst segmentation with inert wall,

(2mm Pt sec. ×8, 4mm Pt sec. ×4, 8mm Pt sec. ×2)Catalyst segmentation with cavities. (2mm Pt sec. ×8 )

ISSUE

Page 10: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Results and discussion

(a) The lack of sufficient heat and fuel concentration in the upstream residual gas could not sustain a

homogeneous reaction behind the catalyst.

Flow velocity: 20 m/sec Equivalence ratio: 0.6

Effects of catalyst segmentation

(b)

(c)

(a)

(d)

(16mm Pt sec. ×1)

(8mm Pt sec. ×2)

(4mm Pt sec. ×4)

(2mm Pt sec. ×8)

Exposure time: 1/40 s-1

Page 11: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Results and discussion

(b)&(c) The gas reaction is sustained in the non-catalytic walls adjacent to catalyst segments, where the

gas inherits prior catalytically induced exothermicity and intermediate radicals.

Flow velocity: 20 m/sec Equivalence ratio: 0.6

Effects of catalyst segmentation

(b)

(c)

(a)

(d)

(16mm Pt sec. ×1)

(8mm Pt sec. ×2)

(4mm Pt sec. ×4)

(2mm Pt sec. ×8)

Exposure time: 1/40 s-1

Page 12: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Results and discussion

(d) The upstream catalyst does not provide sufficient catalyst bed to produce catalytically induced exothermicity

for supporting the downstream homogeneous reaction.

Flow velocity: 20 m/sec Equivalence ratio: 0.6

Effects of catalyst segmentation

(b)

(c)

(a)

(d)

(16mm Pt sec. ×1)

(8mm Pt sec. ×2)

(4mm Pt sec. ×4)

(2mm Pt sec. ×8)

Exposure time: 1/40 s-1

Page 13: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Results and discussion

A sufficient length of catalyst segment is an important parameter since it can release sufficient heat to sustain homogeneous reaction in the downstream for preventing thermal and radical quenching on the non-catalytic wall.

Flow velocity: 20 m/sec Equivalence ratio: 0.6

Effects of catalyst segmentation

(b)

(c)

(a)

(d)

(16mm Pt sec. ×1)

(8mm Pt sec. ×2)

(4mm Pt sec. ×4)

(2mm Pt sec. ×8)

Exposure time: 1/40 s-1

Page 14: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Results and discussion

Hydrogen consumption closes to the catalyst surface.

Few OH radical appears in the vicinity of non-catalytic wall adjacent to catalyst segments.

(b) H2

OH

(a)

(c)

Flow velocity: 20 m/sec Equivalence ratio: 0.6

Effects of catalyst segmentation

1.7E-15

1E-16

1.9E-2

0.1E-2

Page 15: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Results and discussion

Heterogeneous reaction

Effects of catalyst segmentation

8 segments with 2mm catalystFlow velocity: 20 m/secEquivalence ratio: 0.6

Kinetically controlled : surface concentration > 95% Mass transfer controlled : surface concentration < 5% The length of catalyst is not long enough to develop into mass-

transfer-control region.

(Pfefferle, 1977)

Page 16: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Results and discussion Effects of catalyst segmentation

Some fluctuation in fuel concentration distributions appear in non-catalytic section, where fuel has no heterogeneous consumption but concentration accumulates due to diffusion from main stream.

Heterogeneous reaction

8 segments with 2mm catalystFlow velocity: 20 m/secEquivalence ratio: 0.6

Page 17: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Results and discussion

Heterogeneous reaction and fuel conversion are completed behind the fourth catalyst segment.

(c) Induced homogeneous reaction behind the third catalyst segment accelerates hydrogen conversion.

Flow velocity: 20 m/sec Equivalence ratio: 0.8

Effects of catalyst segmentation(a)

(b)

(c)

H2

OH8.5E-2

0.5E-2

2.4E-2

0.2E-2

Page 18: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Results and discussion Effects of catalyst segmentation

Heterogeneous reaction takes place in the upstream catalyst segments Downstream catalyst segment inherits thermal energy and thus

induces catalytically supported homogeneous combustion.

Heterogeneous reaction

Homo- and Heterogeneous reaction

8 segments with 2mm catalystFlow velocity: 20 m/secEquivalence ratio: 0.6

Page 19: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Results and discussion

(a)

A wider range for hetero- and homogeneous reactions in the case of 4 catalyst segments and 8 catalyst segments.

(b)

The range for hetero- and homogeneous reactions become wider in the case of eight segments. (A) ER = 0.6 and (B) ER = 0.8.

T h e len g th o f a sin g le ca ta ly st seg m en t (m m )

0

10

20

30

40

50

Inle

t vel

ocit

y (m

/s)

G as and surface reactionSurface reactionF lash back

0

10

20

30

40

50

2 4 8 16

(A)

(B )

Effects of catalyst segmentation

Page 20: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Results and discussion

15 to 30 m/s

Flame anchoring position recedes from the upstream segment to the downstream segment.

35&40 m/s

Gas-phase reaction could not sustain and only surface reaction exists.

Equivalence ratio: 0.8

Segment catalyst with cavities

V=15 m/s

V=20 m/s

V=25 m/s

V=30 m/s

V=35 m/s

V=40 m/s

Effects of catalyst segmentation

Page 21: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Results and discussion

Hydrogen combustion is strongly related to fuel concentration, inflow velocity, and catalyst length, which influence the thermal balance between catalytically induced heat release and heat losses to the wall.

Equivalence ratio: 0.8

Segment catalyst with cavities

V=15 m/s

V=20 m/s

V=25 m/s

V=30 m/s

V=35 m/s

V=40 m/s

Effects of catalyst segmentation

Page 22: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Results and discussion

(b)&(c)

Heterogeneous reaction at the first catalyst segment delivers radicals and heat to the first cavity, and promotes the homogeneous reaction in the cavity.

(d)

Cavities enhance the stabilization of homogeneous reaction by providing a low-velocity region.

Flow velocity: 20 m/sec Equivalence ratio: 0.4

Effects of catalyst segmentation with cavitiesH2

OH

(b)

(c)

(a)

(d) OH

1.1E-2

0

2.0E-3

0

2.0E-3

0

Page 23: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Results and discussion Effects of catalyst segmentation with cavities

Cavity not only provides a low-velocity region for flame stabilization, but also supplies heat to enhance the catalytic combustion.

Heterogeneous reaction

Homo- and Heterogeneous reaction

8 segments with 2mm catalyst and cavityFlow velocity: 20 m/secEquivalence ratio: 0.4

Page 24: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Results and discussion

It appears that the flame is anchored at the first cavity for V 15 m/s.

The unreacted hydrogen could induce homogeneous reaction in the downstream cavities as the inflow velocity is increased.

Equivalence ratio: 0.4

Effects of catalyst segmentation with cavities

Page 25: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Results and discussion

Heterogeneous reaction in the upstream catalyst segment.

Homogeneous reaction in the upstream cavity.

Experimental and numerical results :

cavities appreciably extends the stable operating range in a wide range of inflow velocities.

2mm Pt x8 (A) without (B) with cavities

Effects of catalyst segmentation with cavities

Page 26: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Heterogeneous reaction occurred in the prior catalyst segment generates active chemical radicals and catalytically induced exothermicity; a homogeneous reaction is subsequently induced and anchored in the following non-catalytic wall or cavity.

In comparison with single catalyst, multi-segment catalyst exhibits better performance. However, the better segmentation is strongly related to inflow rate and fuel concentration.

Cavities can collect radicals and hot gas from upstream and provide a low-velocity region to sustain and anchor gas-phase reactions in a wide range of inflow velocities.

In the case of catalyst segmentation with cavity the interaction between hetero- and homogeneous reaction is more like cooperation not competition.

Conclusions

Page 27: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Thank you for your attentionThank you for your attention !!!!

Page 28: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities
Page 29: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Surface reaction vs gas reaction

Exits temperature : Gas reaction (Homogeneous reaction)

>1000K

Surface reaction (Heterogeneous reaction)≤600K

Non reaction ≈300K

Page 30: Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities

Catalytic combustion Heterogeneous reaction

Kinetic control

Mass transfer control region

Transition to mass transfer control

Surface mass fraction (%) >95% Kinetic control < 5% Mass transfer control region

(Pfefferle, 1977)


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