Combustion and Flame 159 (2012) 1644–1651

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Combustion and Flame

journal homepage: www.elsevier .com/locate /combustflame

Effects of catalyst segmentation with cavities on combustion enhancementof blended fuels in a micro channel q

Yueh-Heng Li a,b,⇑, Guan-Bang Chen a, Fang-Hsien Wu b, Tsarng-Sheng Cheng c, Yei-Chin Chao b,⇑a Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 701, Taiwan, ROCb Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan 701, Taiwan, ROCc Department of Mechanical Engineering, Chung Hua University, Hsinchu 300, Taiwan, ROC

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 July 2011Received in revised form 31 October 2011Accepted 25 November 2011Available online 28 December 2011

Keywords:Numerical simulationCatalytic combustionCatalyst segmentationCavityMicro-reactorSyngas

0010-2180/$ - see front matter � 2011 The Combustdoi:10.1016/j.combustflame.2011.11.017

q This paper is submitted for publication in Combusthis manuscript have neither been published inpreviously, nor will it be submitted to another journalCombustion and Flame.⇑ Corresponding authors. Addresses: Research Cent

Strategy, National Cheng Kung University, No. 1, Ta-HROC. Fax: +886 6 2095913 (Y.-H. Li), Department ofNational Cheng Kung University, No. 1, Ta-Hsueh Rd., T+886 6 2389940 (Y.-C. Chao).

E-mail addresses: yuehheng.li@gmail.com (Y.-H. L(Y.-C. Chao).

A novel design concept for combustion enhancement of H2/CO, CH4/CO, and H2/CH4 blended fuels in amicro channel using combined effects of catalyst segmentation and cavities is proposed. The enhance-ment and combustion characteristics are evaluated by numerical simulation with detailed heterogeneousand homogeneous chemistries. Effects of unsegmented and segmented catalysts with and without cavi-ties are examined and discussed in terms of different multi-fuel mixtures. In general, it is found that thechemical process of conventional catalytic combustion is a competition for fuel, oxygen, and radicalsbetween heterogeneous and homogeneous reactions. On the other hand, the purpose of using catalystsegmentation and cavities in a micro-reactor is to integrate advantages of heterogeneous and homoge-neous reactions, to enhance fuel conversion, and to promote complete combustion in a short distance.In the proposed catalyst configuration, the heterogeneous reaction in a prior catalyst segment produceschemical radicals and catalytically induced exothermicity, and the homogeneous reaction can be subse-quently ignited and anchored in the following cavity. H and OH radicals from both hydrogen and meth-ane may obviously change the chemical pathway of CO oxidation. Full multi-fuel conversion andcomplete combustion can thus be achieved in a short distance. The existence of cavities appreciablyextends the stable operational range of the micro-reactor for a wide range of inlet flow velocities. More-over, cavities in a small-scale system can further stabilize the flame, and serve as a heat source to enhancethe reaction. These features allow the proposed catalyst configuration to apply to various small-scalepower, heat generation and propulsion systems.

� 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction

Hydrocarbon-fueled micro-reactors have received increasingattention for electrical power generation in portable electronicsdue to their superior energy density (45 KJ/g) compared to thatof state-of-the-art lithium batteries (1.2 KJ/g) [1]. The conversionof the chemical energy of fuels into electricity in a micro-scaledevice without moving parts can be achieved using fuel cells,

ion Institute. Published by Elsevier

tion and Flame. Materials innor submitted to a journalduring the review process of

er for Energy Technology andsueh Rd., Tainan 701, Taiwan,

Aeronautics & Astronautics,ainan 701, Taiwan, ROC. Fax:

i), ycchao@mail.ncku.edu.tw

photovoltaics [2,3], and thermoelectrics [4]. A major threshold inpractical micro-reactors (with characteristic dimensions <1 mm)is their enhanced heat loss and combustion instability. Homoge-neous flames are typically quenched when confined in spaces withdimensions below their quenching distances [5].

The increased surface-to-volume ratio of micro-reactors leadsto thermal and radical quenching of reactions. Catalytic micro-combustor exhibits wider stability than homogeneous micro-com-bustor [6,7]. The catalytic layer deposited on the reactor walls maysustain chemical reactions at lower temperatures and in the pres-ence of higher heat losses, thus reducing the impact of thermalquenching. However, complicated heterogeneous–homogeneousinteractions can be observed in catalytic micro-reactors. Well-know aspects of these interactions include the promotion of gas-phase reactions due to catalytically induced exothermicity andthe inhibition of gaseous reactions caused by the competition offuels and oxidizers of the catalyst bed versus gas phase reactions.The competition between heterogeneous and homogeneous reac-tions often leads to incomplete combustion and a narrowing ofthe stable operating range. Nevertheless, some strategies have thus

Inc. All rights reserved.

Fig. 1. Schematic of computational domain.

Y.-H. Li et al. / Combustion and Flame 159 (2012) 1644–1651 1645

been proposed for micro-reactors, such as heat recuperation [8,9]and utilizing quench-resistant fuels [10], etc.

Undoubtedly, a proper combustor configuration [11–13] and awell designed catalyst bed [14,15] can improve the reaction andreduce heat and radical loss. Federici et al. [8] demonstrated thatthe thermal properties of reactor materials play a vital role inthe overall thermal stability of micro-reactors. The reactor wallsnot only contribute to heat loss through conduction, but they areoften responsible for the majority of heat transfer from upstream,which is necessary to preheat the feed to ignition temperature. Inother words, a properly maintained high wall temperature inlocalized space can extend the gas reaction and mitigate radicalquenching. Li et al. [11] and Benedette et al. [13] reported that ahybrid micro-combustor consisting of alternating catalytic andnon-catalytic segment have been designed to operate under highvelocities to overcome the blow-out regions or to selectively pro-mote homogeneous combustion to improve the overall conversionin the system.

Recently, most research activities in the field of catalytic com-bustion for energy production has emphasized on the use ofmethane, mainly for its nature of clean fuel, widely availableworld-wise. Nonetheless, methane is inherently the most stable,difficult-to-oxidize hydrocarbon with outstanding characteristicsthat its catalytic ignition occurs at relatively high temperatureeven on the most active and expensive PdO-based catalysts.Some strategies were proposed to overcome the shortcoming,such as preheating the fuel–air gas, adding hydrogen in meth-ane-fuel and proposing new catalyst layout to improve hetero-and homo-geneous reaction in a confined space. In our previous[16], we proposed a configuration of catalyst segmentation withcavity to accelerate the methane conversion in a microreactor.The details in the correlation between heterogeneous and homo-geneous reaction in different catalyst layouts are briefly ad-dressed in the following section. Furthermore, Westbrook andDryer [17] identified heterogeneous–homogeneous radical cou-pling in methane combustion over platinum tubes. The gas-phase combustion of methane can be roughly described by atwo-step process, the incomplete oxidation of CH4 to CO andthe main heat-releasing oxidation of CO to CO2. However, carbonmonoxide is certainly active on the platinum surface due to itshigh sticking coefficient (0.85 on Pt). By depriving CO from thegas-phase, the catalyst inhibits the homogeneous reaction ofCO. Accordingly, mixing methane with CO and H2, such as com-position of the syngas and gasified biomass [18], might beadvantageous due to the higher reactivity of such compoundswhich could facilitate star-up [19] and/or stabilize the catalyticreaction without the need of further pre-heating or piloting[20]. When burning multi-fuels of carbon monoxide and hydro-gen, blending with hydrocarbon in a catalytic micro-reactor willexhibit complicated heterogeneous and homogeneous reactionsdue to their different diffusive and catalytic characteristics. Nev-ertheless, the interplay of kinetics and transport of CH4/CO/H2

multi-fuel reactions in catalytic micro-reactors have rarely beenstudied.

The utilization of micro-combustor requires high power designdepending on purpose, which can be obtained by increasing the in-let mass flow rate and thus gas velocity. On the contrary, to reachhigh conversions, the residence time should be relatively highwhich means that low inlet gas velocities are needed to preventblow-out. As a result, a trade-off in the choice of inlet gas velocitieshas to be reached. Traditionally, the stable operating range of amicro-reactor is restricted to low inlet velocities (less than 1 m/s)[7–10]. To extend the operation range and to study the interplayof H2/CO/CH4 multi-fuel reactions in a catalytic micro-reactor� anovel catalyst bed design that uses catalyst segmentation withcavities is proposed in the present study.

2. Numerical model and chemical mechanism

In this work, a commercial code, CFD-ACE [21], was modifiedand incorporated with detailed gas-phase and surface reactionmechanisms in CHEMKIN format to simulate the flow and reactioncharacteristics inside a micro channel. For simplicity, the micro-reactor was modeled as a two-dimensional system in the numeri-cal simulation, with a gap width (L) of 1 mm between the twoparallel plates. In practical applications, a micro-reactor with alarge aspect ratio can be fabricated using MEMS technology. Thegoverning equations consist of two-dimensional Navier–Stokesequations, mass and energy conservation equations, and a speciesequation for each chemical species. Figure 1 shows a schematic ofthe catalytic micro channel modeled in this work. Simulationswere performed on half of the channel due to symmetry. The reac-tor was 3 cm in length and had a wall thickness of 0.2 mm. Cavitieswere used to increase the residence time and to enhance thehomogeneous reaction. The catalyst segments were coated withplatinum. A total catalyst length of 1 cm was used for segmentsof various lengths in the comparisons. The cavity width was1 mm and the cavity depth was 0.2 mm.

For boundary conditions, the stoichiometric fuel–air mixturewas specified at the inlet. Three multi-fuel compositions werestudied: 50%H2 + 50%CO, 50%CH4 + 50%CO, and 50%CH4 + 50%H2.The inlet temperature for the fuel/air mixture was 300 K. A uni-form velocity profile of 10 m/s was specified at the inlet. A laminarflow field was used for all cases. The thermal boundary condition atthe wall was the heat lost to the ambient air at 300 K. The exteriorheat loss was due to heat convection by air, described as:

q00 ¼ hðTw � 300Þ ð1Þ

where h is the heat transfer coefficient (20 W/m2/K in this study)and Tw is the wall temperature. At the exit, the pressure was spec-ified as a constant ambient pressure of 101 kPa and an extrapolationscheme was used for species and temperature.

Non-uniform meshes were used with more grids distributed inthe reaction region to provide sufficient grid resolution in the com-putational domain. Grid independence was examined and a non-uniform mesh with a distribution of 211 � 65 grid points in the ax-ial and transverse directions was used. The simulation convergedwhen the residuals of all governing equations approached steadystates with residuals smaller than 10�4.

For gas properties and transport coefficients, the mixture den-sity was calculated using the ideal gas law and the mixture viscos-ity, specific heat, and thermal conductivity were calculated from amass average of species properties. Detailed gas-phase andcatalytic surface reaction mechanisms were applied. The reactionrate was represented by the modified Arrhenius expression. Amodified Arrhenius expression can be introduced by expressingk(T) = BTa Exp(�Ea/RT), where B is a constant, Ea is the activationenergy of the reaction (J/mole) and a is temperature exponent.The GRI-Mech 3.0 mechanism was used for gas phase reactions;it comprises 53 species and 325 reaction steps. The surface reac-tion mechanism was compiled primarily from that proposed byDeutschmann et al. [22]. These reaction mechanisms have beenused in previous studies and comparisons with experimental re-sults were satisfactory [23,24]. For methane fuel, eleven surface

1646 Y.-H. Li et al. / Combustion and Flame 159 (2012) 1644–1651

species (C(s), CH(s), CH2(s), CH3(s), CO(s), CO2(s), H(s), H2O(s), O(s),OH(s), PT(s)) describe the coverage of the surface with adsorbedspecies. PT(s) denotes free surface sites available for adsorption.

Y(m

m)

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0.9 OH6.0E-03

0.0E+00

CH40.0338

3. Results and discussion

3.1. Effects of catalyst configuration

An investigation of methane combustion in a catalyst channelwas conducted first since methane is typically weak in hetero-/homo-geneous coupling and requires a long catalyst length forcomplete conversion over platinum [25,26]. In our previous paper[16], three kinds of micro-reactor design were implemented to im-prove the methane conversion: the catalytic system with a singlecatalyst sector, system with catalyst segmentation, and systemwith catalyst segmentation and cavities. The OH radical is one ofsignificant radicals in the hydrocarbon oxidation, and the existenceof OH radical is generally indicated the reaction zone and hightemperature regions. Consequently, OH mass fraction is usuallyused to delineate the gas reaction in a catalyst combustion[22,25]. Figure 2 shows the computed results of methane and OHmass fraction contours for various catalyst configurations for theentire section. The equivalence ratio of these cases is 0.6 and theinlet velocity is 10 m/s. This velocity substantially exceeds theflame speed of methane; homogeneous combustion cannot besustained in a micro-reactor with non-catalytic walls under thiscondition. However, the catalyst on the wall evidently extendsthe blowout limit of methane. Numerical results indicate that thehomogeneous combustion exists in the centerline for all cases.The corresponding flame anchoring positions, as shown by the highOH concentration regions, are apparently distinct. In the singlecatalyst case, fuel is consumed partially by the heterogeneous reac-tion in the vicinity of the catalyst and the other is consumed by the

Y(m

m)

-0.9

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

(a)

Y(m

m)

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0

0.9

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Axial distance (cm)

Y(m

m)

0 0.5 1 1.5 2 2.5 3-0.9

0

0.9

(c)Fig. 2. Color-coded contours of methane and OH mass fraction for various catalystconfigurations: (a) without segmentation and cavity, (b) with 2 mm � 5 segmen-tation, and (c) with 2 mm � 5 segmentation and cavities.

homogeneous reaction in the centerline region. The catalytic sur-face supplies heat, intermediate species, bare active sites or others,and sustains the gas reaction in this region.

For the multi-segment catalyst case, homogeneous combustionis sustained in spaces between adjacent catalyst segments, wherethe mixture inherits prior catalytically induced exothermicityand intermediate species. High flow velocity moves the flameanchoring downstream of the second catalyst segment. Methanedepletion occurs within the first two catalysts, and surface chem-istry dominates the local reaction in this region. Subsequently, alarge number of radicals congregated on the non-catalyst wall,where homogeneous chemistry dominates the local reaction, asshown in Fig. 2b. For the case of multi-segment catalyst withcavities, methane attains complete conversion in a short distance,and the flame anchoring moves upstream, as shown in Fig. 2c. Theexistence of cavities can provide the low velocity zone to stabilizethe gas reaction, so that the methane oxidation can be acceleratedby assisting homogeneous and heterogeneous reaction. The perfor-mance of integrated catalyst segmentation with interlacingcavities is superior to that of the single catalyst, especially inhigh-velocity flows.

-0.9 0

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m)

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0 0.2 0.4 0.6 0.8 1

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OH6.0E-03

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CH40.0338

0

Fig. 3. Velocity magnitude and CO mass fraction contours superposed on color-coded OH and CH4 mass fraction for the first 1.0 cm section of various catalystconfigurations: (a) without segmentation and cavity, (b) with 2 mm � 5 segmen-tation, and (c) with 2 mm � 5 segmentation and cavities.

Y(m

m)

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0.9 H20.0120.0090.0060.0030.000CO0.162

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m)

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H20.0120.0090.0060.0030.000

(a)

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0 0.5 1 1.5 2 2.5 3

0 0.5 1 1.5 2 2.5 3

Y.-H. Li et al. / Combustion and Flame 159 (2012) 1644–1651 1647

In order to further explore the hetero- and homo-geneous inter-action in various catalyst configurations, Fig. 3 shows significantspecie and radical distributions along the first 1.0 cm section ofthe micro channel. The computed velocity magnitude, CO, CH4

and OH mass fractions expressed in iso-velocity, iso-concentrationand color1 codes of methane catalytic combustion in the micro-channel with a single 10-mm-long catalyst was displayed inFig. 3a. The result shows that methane is depleted in the catalystbed via the heterogeneous reaction, and the following homoge-neous reaction, which anchors in the boundary layer, is sequen-tially induced. The OH radicals are congregated in the centerlineof the channel, whereas the radicals near the catalyst bed are sub-ject to absorption by the catalyst bed due to the high sticking coef-ficient of OH radicals. It leads to deprive the homogeneous reactionof OH radicals. This phenomenon is related to specie quenching,namely the competition of hetero- and homogeneous reactions.Besides, the contour of the CO mass fraction in Fig. 3a demon-strates the formation of carbon monoxide behind the homoge-neous reaction, and it occurs by incomplete combustion. Figure3b shows methane catalytic combustion in the micro channel withcatalyst segmentation (five segments of the 2-mm catalyst). It dis-plays that the heterogeneous reaction occurs in first two catalystsegments, and the majority of methane conversion is achieved infirst two catalyst segments. The OH mass fraction distribution de-notes the location of the gas reaction in the centerline of the chan-nel. The space between adjacent catalyst segments provides aproper dwelling place for sustaining a homogeneous reaction withsufficient intermediate species and catalytically induced exother-micity from upstream. It appears that flames are anchored on thesespaces, and the flame behavior is similar to the results of Bened-etto’s work [13]. The increase of the CO mass fraction in Fig. 3b re-veals incomplete combustion of the homogeneous reaction in highvelocity conditions, and the decrease of the CO mass fraction afterthe following catalyst segments is caused by its high sticking coef-ficients on Pt surface. The catalyst segmentation effectively inte-grates the hetero- and homogeneous reactions to accomplishcomplete methane conversion in a short length in a micro-reactor.However, the flames which stabilize on the spaces are prone to beaffected by gas velocity.

High inlet velocity may reduce the residence time and deferthe onset of homogeneous ignition downstream. In order toenhance flame stabilization in the micro-reactor, localized cavitieson the channel wall are proposed. The function of cavities is to pro-vide a low-velocity zone to stabilize the homogeneous reaction.Figure 3c shows that the gas velocity magnitude inside the cavityis relatively low compared to that in the main stream; that is, cav-ities can stabilize the homogeneous reaction by providing a low-velocity asylum. The CO mass fraction distribution illustrates thereaction of the fuel mixture in a prior catalyst segment, and thecongregation of OH radicals in the cavities represents the flameanchoring. The distance for complete methane depletion is reducedand the flame anchoring location moves upstream compared tothat shown in Fig. 3a and b. As to the effects of inlet velocity, seg-ment catalyst layout and cavity dimension, the further discussionsof methane combustion in a micro-scale catalytic channel are pre-sented in the previous paper [26].

3.2. Issues of fuel composition

Generally, gasified biomass, syngas and industrial residual gas[27,28] contain various amount of hydrogen, carbon monoxideand light hydrocarbons. However, these fuels, such as H2, CO and

Fig. 4. Computed contours of H2, CO (color coded), OH and CO2 mass fraction withan equivalence ratio of 1.0 and an inlet velocity of 10 m/s for (a) single catalyst, (b)multi-segment catalyst and (c) multi-segment catalyst with cavities.

1 For interpretation of color in Figs. 1–12, the reader is referred to the web versionof this article.

CH4, have distinct physical and chemical characteristics in hetero-and homogeneous reactions. In order to investigate the interplay ofthese fuels over platinum catalyst, binary fuels among H2, CO andCH4 were addressed and compared for the hetero- and homoge-neous reactions of three catalyst configurations in the stoichiome-tric condition. The composition ratio of binary fuel is considered as50–50% mixture and its corresponding inlet velocity is fixed at10 m/s for all cases.

3.2.1. Hydrogen and carbon monoxide mixtureHydrogen and carbon monoxide have relatively high sticking

coefficients to platinum catalyst. However, hydrogen has an inher-ently large mass diffusivity compared to that of carbon monoxide,so hydrogen first reaches the catalyst bed and triggers the hetero-geneous reaction. Figure 4 shows the computed contours of fueland the intermediate species mass fractions for stoichiometricmixtures of an inlet velocity of 10 m/s for micro channels with con-figuration of single catalyst, catalyst segmentation and catalystsegmentation with cavities. For the single catalyst configuration,hydrogen inherently triggers heterogeneous reaction on the cata-lyst section, approximately reaches complete conversion and thensuccessively induces the gas reaction behind the catalyst. The gasreaction mainly performs the fuel conversion of carbon monoxide,so that the carbon monoxide conversion in a 30-mm micro-chan-nel is about 83%. Nevertheless, in the cases of a multi-segment cat-alyst with and without cavities no significant difference betweenthe two catalyst configurations was observed. Similarly, the gasreaction was anchored in the inert walls or cavities between adja-cent catalyst segments. The hydrogen and carbon monoxide con-version for both configurations can reach to 97% and 90%,respectively, which is definitely higher than that in a single catalystcase. In order to understand the interaction between hetero- andhomo-geneous reactions in a single catalyst case, Fig. 5 showsthe ratio of surface mass fraction to mean bulk mass fraction along

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

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50

60

70

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90

100Y s

/Yb

(%)

CO mass fractionH2 mass fraction

Heterogeneous reaction

Homogeneous reaction

5%

95%

Catalyst section

Fig. 5. Ratio of surface mass fraction to mean bulk mass fraction of reactant alongthe single catalyst micro-channel.

1648 Y.-H. Li et al. / Combustion and Flame 159 (2012) 1644–1651

the channel. In general, the heterogeneous reaction can be consid-ered as kinetically controlled, for which the surface concentrationis greater than 95% of the bulk concentration, and as mass transfercontrolled, for which the surface concentration is less than 5% ofthe bulk. Within the 10 mm-long catalyst bed, hydrogen is com-pletely consumed, but carbon monoxide has no significant reac-tion. It appears that hydrogen tends to induce heterogeneousreaction over the Pt catalyst, but the length of catalyst section isnot enough to develop into mass-transfer-control region. Hetero-geneous reaction of carbon monoxide is barely observable and itbelongs to kinetic-control region in the catalyst section. However,induced homogeneous reaction behind the catalyst prompts fur-ther acceleration of hydrogen/carbon monoxide chemical reac-tions. It turns out that carbon monoxide conversion dominantlycounts on homogeneous reaction, while hydrogen conversionequally relies on heterogeneous and homogeneous reactions. Con-sequently, the fuel conversion efficiency in single catalyst micro-channel would be strongly related to fuel concentration, fuel flowrate, fuel composition and catalyst length.

Figure 6 shows the fuel mass fraction distribution along the ax-ial direction close to the inner wall for three different catalyst con-figurations. For multi-segment catalyst without and with cavities,hydrogen tends to lightoff early on the catalyst surface, providingcatalytically induced exothermicity and significant amounts of

0 0.5 1 1.5 2 2.5 3

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0

0.002

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0.014

H2 m

ass

frac

tion

0

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CO

mass fraction

Single catalyst caseSegment catalyst caseSegment catalyst with cavities case

H2 CO mass fraction

Fig. 6. Fuel (50%H2 and 50%CO) mass fractions for single catalyst, multi-segmentcatalyst and multi-segment catalyst with cavities.

chemical radicals to support the downstream hetero-/homoge-neous reaction of carbon monoxide. It shows the effects to movethe carbon monoxide reactions upstream. Some bumps in fuel con-centration distributions appear in non-catalyst section, where fuelhas no heterogeneous consumption but concentration accumulatesdue to diffusion from mean stream. The Lewis number of hydrogenis about 0.346, which is much smaller than that of CO (1.376). Thismeans that hydrogen is responsible for initializing surface reactionand stabilizing the gas reaction in high flow velocity due to its highmass diffusivity, and carbon monoxide is charged for main heat re-lease in homogeneous reaction due to the fact that most hydrogenis consumed in catalyst section.

In general, syngas and gasified biomass have varying amountsof fuel compositions on hydrogen and carbon monoxide. In orderto investigate the fuel properties issues in the multi-segment cat-alyst with cavities system, Fig. 7 shows the fuel mass fraction dis-tributions along the axial direction close to the inner wall for casesof various fuel compositions, 30%H2 + 70%CO, 50%H2 + 50%CO and70%H2 + 30%CO, respectively. Results show no significant differ-ence among these conditions, and have certainly high fuel conver-sions. Proper hydrogen amount in fuel mixture benefits to promptthe hetero- and homogeneous reactions of carbon monoxide in themicro channel.

3.2.2. Methane and carbon monoxide mixtureMethane and carbon monoxide have similar Lewis numbers, but

CO has a high sticking coefficient to platinum. Nevertheless, themaximum laminar burning velocity increases with increasing COcontent in the CH4-air mixture, and fuel–air mixture leans towardfuel-rich side [29]. For example, laminar burning velocity of50%CH4 + 50%CO fuel mixture in stoichiometric condition increasesby 30% than that of pure methane. In the single catalyst case, theCH4/CO mixture cannot stabilize in high flow velocity. However, amulti-segment catalyst sustains the gas reaction in the micro chan-nel. Figure 8 shows that methane becomes depleted in the first twocatalyst segments. However, incomplete combustion yields a highconcentration of CO species. The upstream heterogeneous reactionprovides chemical radicals and exothermicity, and the homoge-neous reaction speeds up the dissociation of fuel species. However,the very short residence time leads to a high carbon monoxide con-centration in the exhaust gas. Subsequently, the following sequen-tially-segmented catalyst helps deplete carbon monoxide in theresidual combustible mixture until a complete conversion. For thecase with cavities, it shows that the gas reaction moves upstream

0 0.5 1 1.5 2Axial distance (cm)

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120

Y s/Y

b(%

)

30%H2+70%CO50%H2+50%CO70%H2+30%CO

H2 CO mass fraction

Fig. 7. Fuels (H2 and CO) mass fractions for various fuel compositions in a multi-segment catalyst with cavities within the first 2 cm from the entrance.

Y(m

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1 2

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OH8.0E-036.0E-03

4.0E-03

2.0E-03

0.0E+00

(a)

(b)

(c)Fig. 8. Computed contours of (a) CH4, (b) CO, and (c) OH mass fractions with anequivalence ratio of 1.0 and an inlet velocity of 10 m/s for multi-segment catalyst(top) and multi-segment catalyst with cavities (bottom).

Axial distance (mm)

Mas

sfra

ctio

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Segment catalyst without Segment catalyst with cavities

Segment catalyst without Segment catalyst with cavities

Fig. 9. (a) Fuel (50%CH4 and 50%CO) and (b) radical mass fraction for multi-segmentcatalyst and multi-segment catalyst with cavities within the first 2 mm from theentrance.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Axial distance (cm)

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20%CH4+80%H2

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50%CH4+50%H2

H2 CH4 mass fraction

Fig. 10. Fuel (H2 and CH4) mass fractions for various fuel compositions in a multi-segment catalyst within the first 2 cm from the entrance.

Y.-H. Li et al. / Combustion and Flame 159 (2012) 1644–1651 1649

and anchors in the first cavity. Furthermore, a shorter distance is re-quired for a relatively high conversion.

Figure 9 shows the fuel and radical mass fraction distributionsin the first 2 mm from the entrance. In the first 1 mm, methane be-comes depleted in the catalyst segment, yielding CO and CH3 spe-cies. In the next 1 mm, the OH distribution in the cavity implies agas reaction, so CH4 and CH3 are further depleted in this region. Ahigh flow velocity postpones the onset of the gas reaction in themulti-segment catalyst case, and the increase of the methane massfraction along the axial distance in the figure is due to mass diffu-sion from the main stream. Although methane may also yields car-bon monoxide if incompletely reacted, OH and H radicals from thegas reaction may promote CO depletion. The fast chemical reactionof CO + OH ? CO2 + H for the gas reaction is more active than the‘‘dry CO oxidation’’ reaction of CO + O2 ? CO2 for the surface reac-tion. In other words, the CO surface reaction dominates in the firstcatalyst segment, providing exothermicity to pre-react methane.Methane releases H and OH radicals to assist CO conversion inthe following gas reaction.

3.2.3. Hydrogen and methaneFor hydrogen and methane mixture, there is no obvious gas

reaction in the single and multi-segment catalyst micro-reactorsunder the stoichiometric and inlet velocity of 10 m/s conditions ,but gas reaction can be stabilized in the micro-reactor for the casewith cavities. Comparing with the CH4/CO results above, thehydrogen in CH4/H2 mixture has a similar volumetric energy den-sity (12.7 MJ/m3) with that of CO (11.6 MJ/m3), but hydrogen hashigher mass diffusivity. Therefore, most hydrogen is consumed inthe upstream section of the catalyst. It turns out that catalyticallyinduced exothermicity from hydrogen in the single and multi-seg-ment catalyst cases is unable to ignite the gas reaction of methanedue to low energy density and high heat loss. Hydrogen concentra-tion in CH4/H2 mixture plays an essential role in successfully

triggering of hetero- and homogeneous reactions in a micro chan-nel. Figure 10 shows fuels mass fractions for various hydrogen

Y(m

m)

0 0.5 1 1.5 2 2.5 3-0.9

0

0.9 H20.0050.0040.0030.0010.000

CH40.0440.0330.0220.0110.000

CO

Axial distance (cm)

Y(m

m)

0 0.5 1 1.5 2 2.5 3-0.9

0

0.9OH

8.0E-03

6.0E-034.0E-03

2.0E-03

0.0E+00

H5.0E-044.0E-043.0E-042.0E-041.0E-040.0E+00

Fig. 12. Computed contours of (a) fuel and (b) radical mass fraction with anequivalence ratio of 1.0 and an inlet velocity of 10 m/s for multi-segment catalystwith cavities.

Axial distance (mm)

Tem

pera

ture

(K)

Mas

sfra

ctio

n

0 0.5 1 1.5 21300

1400

1500

1600

1700

1800

0

0.01

0.02

0.03

0.04

0.05

TemperatureCH4H2(x10)

0.05

0.06

1650 Y.-H. Li et al. / Combustion and Flame 159 (2012) 1644–1651

volumetric concentrations in CH4/H2 mixture in a multi-segmentcatalyst case. For the case of 50%H2 + 50%CH4 there is almost nochemical reaction of methane, but hydrogen has obvious fuel con-sumption along the channel. Once hydrogen volumetric concentra-tion in CH4/H2 mixture increases to 70%, methane is consumedperiodically on the catalytic surface very slowly in the downstreamregion of the catalyst segments, but no significant gas reaction canbe found. When hydrogen is increased to 80 vol.% in CH4/H2 mix-ture, the surface reaction of hydrogen can successfully ignite thegas reaction of methane and has complete conversion of methane.The minimum hydrogen volumetric concentration in CH4/H2 mix-ture for complete methane conversion in this specific condition is71 vol.%.

In addition, the thermal conductivity of wall material in the mi-cro channel is another essential parameter to affect hetero- andhomogeneous reactions. Figure 11 shows that fuel mass fractionfor three kinds of wall materials in the multi-segment catalyst case,silicon (124 W/m/K), platinum (69.1 W/m/K) and cordierite(3.3 W/m/K), respectively. These materials are feasible and practi-cally applied in MEMS systems. Results show that high thermalconductivity materials, such as silicon, significantly enhance chem-ical reactions of methane and hydrogen. It means that high thermalconductivity can deliver thermal heat from catalyst section to non-catalyst section for stabilizing gas reaction and even heat up thedownstream catalyst segments for accelerating sequential surfacereaction.

In principal, cavities decelerate the flow in localized spaces andaccumulate radicals from upstream. Figure 12 shows the fuel andradical mass fractions for the multi-segment catalyst with cavities.Reactions in the cavity stabilize the main gas reaction in the chan-nel. As shown by the mass fraction distributions in Fig. 13a, meth-ane is consumed in two stages within 2 mm from the entrance. Thefirst stage, within the first 1 mm, is dominated by the heteroge-neous reaction of the catalyst, while the second stage, in the next1 mm, is responsible for the gas reaction anchoring in the cavity.A sharp temperature rise and OH radical congregation imply agas reaction in the cavity, as shown in Fig. 13b. The amount ofhydrogen decreases in the catalyst section, and then abruptly risesin the cavity section after the dissociation of methane. The CO massfraction has a broadband distribution in two sections. Accordingly,the incomplete combustion of methane leads to a high COconcentration.

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

Y s/Y

b(%

)

Silicom (124 W/m/K)Platinum ( 69.1W/m/K)Cordierite ( 3.3W/m/K)

H2 CH4 mass fraction

Fig. 11. Fuel (50%CH4 and 50%H2) mass fractions for various wall materials in amulti-segment catalyst within the first 2 cm from the entrance.

Axial distance (mm)

Mas

sfra

ctio

n

0 0.5 1 1.5 2-0.01

0

0.01

0.02

0.03

0.04

COCH3OH

Fig. 13. (a) Temperature, fuel (50%CH4 and 50%H2), and (b) radical mass fraction formulti-segment catalyst and multi-segment catalyst with cavities within the first2 mm from the entrance.

The high wall temperature in the high flow velocity case is thekey to sustaining a gas reaction in the micro-reactor. In view of thelow volumetric energy density but high mass diffusivity of hydro-gen, diminishing heat loss and providing a stable heat source on

Y.-H. Li et al. / Combustion and Flame 159 (2012) 1644–1651 1651

the wall are important factors for high chemical conversion. Conse-quently, the hydrogen volumetric concentration in the CH4/H2

mixture and wall properties is an important design parameter.

4. Conclusion

The effects of catalyst segmentation and cavities on H2/CO/CH4

multi-fuel combustion enhancement in a micro-reactor wereinvestigated using numerical simulation with detailed heteroge-neous and homogeneous mechanisms of methane, carbon monox-ide, and hydrogen. Three catalyst configurations were consideredto improve the hetero- homo-geneous reactions in the micro chan-nel. Results reveal that the heterogeneous reaction in the prior cat-alyst segment can produce active chemical radicals andcatalytically induced exothermicity; a homogeneous reaction issubsequently induced and anchored in the following cavity. TheCO/H2 mixture can be sustained in high flow velocity for all threecatalyst configurations due to the high sticking coefficients of COand H2. This allows CO/H2 to lightoff on the catalyst segment.The CO/CH4 mixture can be stabilized in high flow velocity in a mi-cro channel with a multi-segment catalyst and with a catalyst withcavities. In the upstream catalyst segment, incomplete combustionof methane yields carbon monoxide, and the following catalystsegments can then completely consume the yielded carbon mon-oxide due to the preferred CO catalytic reaction of a high stickingcoefficient on the platinum surface. Furthermore, OH and H radi-cals from methane enhance the CO gas reaction by switching thechemical reaction pathway of CO. For the CH4/H2 mixture, onlythe case with catalyst segmentation and cavities can stabilize thegas reaction with high inlet flow velocities. Cavities can collect rad-icals and hot gases from upstream and provide low velocity zone tosustain and anchor gas reactions for high inlet flow velocity mix-tures even though hydrogen provides low volumetric energy den-sity. These processes of multi-fuel catalytic combustion areassociated with the mutual assisting coupling between the hetero-geneous and homogeneous reactions, instead of the competitionfound in the conventional catalyst reactor. A complete methaneconversion and combustion can thus be accomplished in a shortdistance, allowing the system to be further scaled down. The cav-ities appreciably extend the stable operation range of the multi-fuel micro-reactor for a wide range of inlet flow velocities. Thesefeatures allow the proposed catalyst configuration to be appliedto various small-scale power and heat generation systems.

Acknowledgments

The computer time and the numerical packages provided by theNational Center for High-Performance Computing, Taiwan (NCHC-Taiwan), are sincerely acknowledged.

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