Numerical study of heat and mass transfer in the plate methanol steam micro-reformer channels

Numerical study of heat and mass transfer in the plate methanol steam

micro-reformer channels

Ching-Yi Hsueh

a

, Hsin-Sen Chu

a,b

, Wei-Mon Yan

c,*

_{, Chiun-Hsun Chen}

a

_{, Min-Hsing Chang}

d

a_{Department of Mechanical Engineering, National Chiao Tung University, Hsin-Chu 300, Taiwan, ROC} b_{Industrial Technology Research Institute, Chu-Tung, Hsin-Chu 310, Taiwan, ROC}

c_{Department of Greenergy, National University of Tainan, Tainan 700, Taiwan, ROC} d_{Department of Mechanical Engineering, Tatung University, Taipei 104, Taiwan, ROC}

a r t i c l e i n f o

Article history:

Received 12 August 2009 Accepted 2 March 2010 Available online 6 March 2010 Keywords:

Micro-reformer Methanol

Heat and mass transfer Numerical analysis

a b s t r a c t

Effects of geometric and thermo-fluid parameters on performance and heat and mass transfer phenomena in micro-reformer channels were investigated by mathematical modeling. The geometric parameters considered were the channel length, channel height, catalyst thickness and catalyst porosity, while the thermo-fluid parameters included wall temperature, inlet fuel temperature, fuel ratio and Reynolds number. The results of the modeling suggest that the methanol conversion could be improved by 49%-points by increasing the wall temperature from 200
_{C to 260
C. The results also show that the CO}

concentration would be reduced from 1.72% to 0.95% with the H2O/CH3OH molar ratio values ranging from

1.0 to 1.6. The values of parameters that enhance the performance of micro-reformer were identified, such as longer channel length, smaller channel height, thicker catalyst layer, larger catalyst porosity, lower Reynolds number and higher wall temperature.

Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, the proton exchange membrane fuel cell (PEMFC) and the direct methanol fuel cell (DMFC) have become widely used as miniature fuel cells[1]. While the PEMFC has high energy density, it needs to carry enough hydrogen fuel. Therefore, methanol steam micro-reformers are being developed for using with the miniature PEMFCs, to overcome the high risk of carrying a large quantity of hydrogen. Thus, there has been much interest in developing also methanol steam micro-reformers.

Several experiments for methanol steam reformers are currently in progress[2e9]. Various reformer types have been used as the foundation for methanol steam reformer designs, including packed-bed reformers and plate reformers. Several successfully fabricated packed-bed reformers for hydrogen production have been reported [2,3]. Kolb et al. [4] presented that the plate reformers have better performance than the packed-bed reformers due to better heat and mass transfers. Therefore, the channels were patterned on the plate methanol steam reformers by several investigators[5e9]. Their results showed that the plate methanol steam reformers were being developed to produce hydrogen for fuel cell systems.

Recently, many simulation studies have been made of the plate methanol steam reformer [10e21]. There have been various numerical studies of the fluid flow in plate methanol steam reformer channels[10,11]. Kwon et al.[10]investigated the pres-sure and velocity distributions in the micro-reformer channels by using computationalfluid dynamics (CFD). The results show the reformer of 17 parallel micro-channels has a much more uniform velocity distribution than that of 36 parallel micro-channels. A uniform velocity distribution may have a better chemical reaction. A three-dimensional model of a micro-scale reactor to investigate velocity and pressure distributions was developed by Pattekar and Kothare[11]. In order to simplify the analysis, many studies have considered the numerical model of methanol steam reformers, only including energy equation and concentration equations with chemical reaction [12e14]. Kawamura et al. [12] proposed a mathematical heat and mass model to analyze the transport phenomena in the plane methanol steam reformer channels. Kim and Kwon[13]have numerically investigated the inner transport phenomena in the plate methanol steam reformer ducts. The results indicated that a lower inlet feed rate has a better methanol conversion. Varesano et al.[14]used a one-dimensional transient mathematical model to study the transport behaviors in a steam reforming reformer with a burner. Furthermore, the continuity equation, momentum equation, energy equation and species equations with chemical reaction were employed to explore the

* Corresponding author. Tel.: þ886 6 260 2251; fax: þ886 6 260 2205. E-mail address:[email protected](W.-M. Yan).

Contents lists available atScienceDirect

Applied Thermal Engineering

j o u r n a l h o me p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a p t h e r m e n g

1359-4311/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2010.03.002

temperature and gas concentration distributions in the reformer by several researchers [15e21]. Park et al. [15] developed 3-D, quasi-3-D and 1D models to study reformer performance. There was good agreement between the experimental and analytical results. Hsueh et al.[16]employed a numerical channel model to analyze various height and width ratios on the plate micro-reformer performance and reactant gas transport characteristics. The results indicated that a reduction in aspect ratio would improve H2production rate and methanol conversion. Arzamendi

et al. [17] established a micro channel model of the thermal integration of steam reformer and catalytic combustor. The results showed that the short diffusion distance and higher area to volume ratio were obtained by using the micro-reformers. A plate methanol reformer model was proposed by Pan and Wang[18]. Their numerical model accurately predicted the methanol conversion and the gas distributions. Kim and Kown[19] devel-oped a novel reforming channel to study the pressure, velocity, temperature and hydrogen mole fraction distributions in the reformer. The results show that the novel flow field had better performance than the serpentineflow field. Kim[20] developed a micro-reformer model to simulate the conversion and temper-ature distributions in the reformer. The results revealed that the methanol conversion increased with increasing the reformer temperature and decreasing the feed rate. Chen et al.[21]used a mathematical model of the plate-type reformer to investigate the heat and mass transfer in a reformer. The results showed that the CO concentration could reduce with lower temperature, larger H2O/CH3OH molar ratio and aspect ratio. In this work, an attempt

is made to examine the detailedfluid flow, heat and mass transfer coupled with chemical reactions in the plate methanol steam micro-reformer channels.

Although a channel model of the plate methanol steam reformer may be well established in the previous work[16], some of the geometric and thermo-fluid parameters aspects have not been addressed so far. Therefore, the effects of the geometric and thermo-fluid parameters on the plate methanol steam micro-reformer performance and the heat and mass transfer are numer-ically investigated in detail. In this work, a numerical model was developed to study the methanol conversion and local heat and mass transfer in the channel of a micro-reformer. The information reported here would be useful in improving plate methanol steam reformer performance.

2. Model description

Fig. 1 presents a schematic of the two-dimensional channel geometry of the plate methanol steam micro-reformer used in the present work. To simplify the analysis, the following assumptions are made:

(1) Theflow is steady and laminar; (2) The inlet fuel is an ideal gas; (3) Theflow is incompressible;

(4) The catalyst layer is an isotropic porous medium; and (5) Thermal radiation and conduction in the gas phase are

negli-gible compared to convection.

According to the model descriptions and assumptions above, the equations for a two-dimensional channel in the plate methanol reformer system are:

Continuity equation: Nomenclature

Ci concentration of species i (mol m3)

cp specific heat at constant pressure (kJ kg1K1)

D hydraulic diameter (m)

Deff effective mass diffusivity (m2s1)

Dk mass diffusion coefficient (m2s1)

Dp catalyst particle diameter (m)

Ea activation energy (kJ mol1)

H channel height (m)

D

HSR enthalpy of reaction for steam reforming (kJ mol1)

D

HrWGS enthalpy of reaction for the reverse wateregas-shift

(kJ mol1)

keff effective thermal conductivity(W m1K1)

kf fluid phase thermal conductivity (W m1K1)

kp permeability (m2)

ks solid medium thermal conductivity (W m1K1)

k1 pre-exponential factor for steam reforming

k2 pre-exponential factor for the reverse wateregas-shift

k2 pre-exponential factor for the wateregas-shift

L channel length (m) Mi mole fraction of species i

Mw,i molecular weight of species i (kg mol1)

mi mass fraction of species i

p pressure (Pa)

R universal gas constant (kJ kg1K1) Re Reynolds number, Re¼

r

uD/

m

RSR Arrhenius reaction rate coefficient for steam reforming

(mol m3s1)

RrWGS Arrhenius reaction rate coefficient for the reverse

wateregas-shift reaction (mol m3s1) Sc species source term for chemical reaction

Su, Sv momentum source terms for the porous medium in

the x and y directions, respectively. St energy source term for chemical reaction

T temperature (
C) Tw wall temperature (
C)

x, y coordinates (m)

u, v velocity components in the x and y directions, respectively (m s1)

Greek symbols

b

inertial loss coefficient

g

H2O/CH3OH molar ratio

d

1 flow channel height (m)

d

2 catalyst layer height (m)

3

porosity

h

methanol conversion

l

0_i the stoichiometric coefficient for reactant i in reaction

l

00_i the stoichiometric coefficient for product i in reaction

s

tortuosity of the porous medium

m

dynamic viscosity (kg m1s1)

m

mix viscosity of the gas mixture (kg m1s1)

f

ij an auxiliary term in calculating viscosity of gas mixture

r

density (kg m3)

r

s catalyst density (kg m3)

Subscripts

r

vu vxþ vv vy  ¼ 0 (1)

Momentum equation (x-coordinate):

3r

uvu vxþ v vu vy  ¼ 

3

vp vxþ

3m

v2_u vx2þ v2_u vy2 ! þ Su (2)

Momentum equation (y-coordinate):

3r

uvv vxþ v vv vy  ¼ 

3

vp vyþ

3m

v2_v vx2þ v2_v vy2 ! þ Sv (3)

The Suand Svare zero in theflow channel region, and in the

catalyst layer, they are as follows[22]:

Su ¼ 

m

_ku p

b

u

r

2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u2_{þ v}2 p (4) Sv ¼ 

m

_kv p

b

v

r

2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u2_{þ v}2 p (5) where kp ¼ D2 p

3

3 150ð1 

3

Þ2 (6)

b

¼ 3:5ð1 

3

Þ Dp

3

3 (7)

The viscosity of the gas mixture can be calculated from Wilke's mixture rule[23]as follows:

m

_mix ¼ X5 i¼ 1 Mi

m

i P₅ j¼ 1Mj

f

ij (8) where

f

_ij ¼ X i h 1þ

m

_i=

m

_j1=2M_w;j=Mw;i1=4 i2 81þM_w;i=Mw;j1=2 (9) Species equation:
uvmi vx þ v vmi vy  ¼ Deff v 2_m i vx2 þ v2_m i vy2 ! þ ð1 

3

Þ

r

_sSc (10)

In the species equations, midenotes the mass fraction of the i-th

species, including CH3OH, H2O, H2, CO2 and CO. The

Ste-faneMaxwell equations were used to calculate the mass diffusion coefficient[24]. The effective mass diffusivity Deffis expressed as

Deff ¼

3

sDk, where

3

, the porosity of the medium, is expressed as

0.38 and 1.00, respectively, in the catalyst layer and the flow channel. There is no chemical reaction in theflow channel, so Scis

zero in theflow channel. In the catalyst layer where the chemical reaction takes place, the reaction model of Purnama et al.[25]is adopted to describe the source term.

Sc ¼ Mw;iðRSRþ RrWGSÞ 

l

00_i 

l

0_i (11)

Purnama et al.[25]and Agrell et al.[26]proposed that using a Cu/ZnO/AlO3catalyst for methanol steam reforming gives rise to

two main chemical reactions, the steam reforming and the reverse wateregas-shift reactions. They also indicated that CO was gener-ated by the reverse wateregas-shift reaction. Pan and Wang[18] and Purnama et al.[25]presented the following relations for the steam reforming reaction and the reverse wateregas-shift reaction:

CH₃OHþ H2O/ k1 CO₂þ 3H2 (12) CO2þ H24 k2 k2 COþ H2O (13) H L δ2 Thermally Insulated Catalyst Layer y x 0 Tw=Constant δ1 u0, T0, C0 Flow Channel

Fig. 1. Schematic diagram of the system.

Table 1

Parameters used in this work.

Channel length L (m)[5] 3.3 102

Channel height H (m)[5] 2.0 104

Catalyst layer thicknessd2(m) 3.0 105

Flow channel heightd1(m) 1.7 104

Inlet average velocity u0(m s1)[5] 0.266

Inlet average temperature T0(
C)[5] 393

Operation pressure (atm)[5] 1

Activation energy for steam reforming (kJ mol1)[18] 76 Activation energy for reverse wateregas-shift (kJ mol1)[18] 108

Catalyst density (kg m3)[24] 1480

Catalyst thermal conductivity (W m1K1)[24] 0.3

Catalyst layer porosity[11] 0.38

Catalyst permeability (m2_{)[11] 2.379 10}12

Mass diffusion coefficient (m2_s1_{)[24] 6.8 10}5

Table 2

Methanol mole fractions for the various grids. Gridlines x (m)

0.005 0.010 0.015 0.020 0.025 0.030 71 16 0.377 0.309 0.261 0.224 0.194 0.171 91 26 0.378 0.309 0.261 0.224 0.194 0.171 111 41 0.378 0.309 0.261 0.224 0.194 0.173

RSR ¼ k1CCH0:63OHC 0:4 H2O exp
Ea RT  (14) RrWGS¼ k2CCO2CH2exp
Ea RT   k2CCOCH2Oexp
Ea RT  (15)

where the steam reforming reaction is a non-reversible reaction and the reverse wateregas-shift reaction is reversible.

Energy equation:

r

cp
uvT vxþ v vT vy  ¼ keff v 2_T vx2þ v2_T vy2 ! þ ð1 

3

Þ

r

_sSt (16)

In the energy equation, the effective thermal conductivity is:

keff ¼

3

kfþ ð1 

3

Þks (17)

In the energy equation, Stin the channel is zero. The catalyst

layer experiences exothermic and endothermic chemical reactions, so Stcan be described as:

St ¼ ð

D

HSRRSRþ

D

HrWGSRrWSGÞ (18)

The boundary conditions are given at the inlet, outlet, wall, and the interface between the channel and the catalyst layer for the numerical model. For the inlet condition (x_{¼ 0), the inlet boundary} conditions are:

u ¼ u0 (19)

m_i ¼ m0;i (20)

T ¼ T0 (21)

At the channel outlet (x¼ L), fully developed flow is assumed and the boundary conditions of velocity, temperature and mass fraction are expressed as:

vu vx ¼ v ¼ 0; vmi vx ¼ 0; vT vx ¼ 0 (22) 0 20 40 60 80 100 200 210 220 230 240 250 260 L=22mm, H=0.2mm, ₂=30 m, =0.38 L=33mm, H=0.2mm, 2=30 m, =0.38 L=44mm, H=0.2mm, ₂=30 m, =0.38 L=33mm, H=0.1mm, ₂=30 m, =0.38 L=33mm, H=1.0mm, ₂=30 m, =0.38 L=33mm, H=0.2mm, ₂=10 m, =0.38 L=33mm, H=0.2mm, ₂=50 m, =0.38 L=33mm, H=0.2mm, ₂=30 m, =0.28 L=33mm, H=0.2mm, 2=30 m, =0.48

T

w

(

o

C)

(%)

0 5000 10000 15000 20000 25000 30000 200 210 220 230 240 250 260 L=22mm, H=0.2mm, 2=30 m, =0.38 L=33mm, H=0.2mm, 2=30 m, =0.38 L=44mm, H=0.2mm, 2=30 m, =0.38 L=33mm, H=0.1mm, 2=30 m, =0.38 L=33mm, H=1.0mm, 2=30 m, =0.38 L=33mm, H=0.2mm, 2=10 m, =0.38 L=33mm, H=0.2mm, 2=50 m, =0.38 L=33mm, H=0.2mm, 2=30 m, =0.28 L=33mm, H=0.2mm, 2=30 m, =0.48

C

CO

(ppm

)

T

w

(

o

C)

a

b

At the insulated plate (y¼ 0), the velocities and the temperature and mass fraction gradients are zero.

u¼ v ¼ 0; vmi vy ¼ 0;

vT

vy ¼ 0 (23)

At the interface between theflow channel and the catalyst layer (y¼ H 

d

2), the velocities and the temperature and mass fraction

gradients are all continuous.

u_y¼ðHd 2Þþ ¼ uy¼ðHd2Þ;

m

vu vy  _y¼ðHd 2Þþ ¼

m

vu vy  _y¼ðHd 2Þ (24) C_i;y¼ðHd 2Þþ ¼ Ci;y¼ðHd2Þ; Deff;ðHd2Þþ vmi vy   y¼ðHd2Þþ ¼ D_eff;ðHd2Þ vmi vy  _y¼ðH d2Þ ð25Þ T_y¼ðHd 2Þþ ¼ Ty¼ðHd2Þ; keff;ðHd2Þþ vT vy  _y¼ðH d2Þþ ¼ k_eff;y¼ðHd2Þ vT vy  _y¼ðH d2Þ (26) 0 500 1000 1500 2000 2500 3000 1 1.1 1.2 1.3 1.4 1.5 1.6 L=22mm, H=0.2mm, 2=30 m, =0.38 L=33mm, H=0.2mm, 2=30 m, =0.38 L=44mm, H=0.2mm, 2=30 m, =0.38 L=33mm, H=0.1mm, 2=30 m, =0.38 L=33mm, H=1.0mm, 2=30 m, =0.38 L=33mm, H=0.2mm, 2=10 m, =0.38 L=33mm, H=0.2mm, 2=50 m, =0.38 L=33mm, H=0.2mm, 2=30 m, =0.28 L=33mm, H=0.2mm, 2=30 m, =0.48

C

CO

(ppm

)

H2O/CH3OH

(mole/mole)

0 10000 20000 30000 40000 50000 1 1.1 1.2 1.3 1.4 1.5 1. L=22mm, H=0.2mm, 2=30 m, =0.38 L=33mm, H=0.2mm, 2=30 m, =0.38 L=44mm, H=0.2mm, 2=30 m, =0.38 L=33mm, H=0.1mm, 2=30 m, =0.38 L=33mm, H=0.3mm, 2=30 m, =0.38 L=33mm, H=0.2mm, 2=10 m, =0.38 L=33mm, H=0.2mm, 2=50 m, =0.38 L=33mm, H=0.2mm, 2=30 m, =0.28 L=33mm, H=0.2mm, 2=30 m, =0.48

C

CO

(ppm)

H2O/CH3OH

(mole/mole)

a

b

Fig. 3. Effects of geometric parameters and H2O/CH3OH molar ratio on the CO concentration at (a) Tw¼ 200
C and (b) Tw¼ 260
C.

0 50 100 150 200 250 300 0 0.2 0.4 0.6 0.8 1 L=33mm, H=0.1mm, 2=30 m, =0.38 L=33mm, H=0.2mm, 2=30 m, =0.38 L=33mm, H=1.0mm, 2=30 m, =0.38 x/L T( oC) T w=260 T w=200

Fig. 4. Effects of the channel heights on temperature distributions along the centerline of the channel at Tw¼ 200
C and Tw¼ 260
C.

At the heated wall (y¼ H), the velocities and the mass fraction gradient are zero, and the temperature is equal to the wall temperature:

u¼ v ¼ 0; vmi

vy ¼ 0; T ¼ Tw (27)

3. Numerical method

A generalized form of the convectionediffusion transport equation was numerically solved using the commercial CFD program, FLUENTÒ 6.1. The finite difference method uses the control volume formulation with the SIMPLE algorithm to solve the mathematical model. The parameters used in the work are listed in Table 1. The convergence criterions for the normalized residuals for each variable were restricted to less than 106. The effects of the number of gridlines on the numerical results are shown inTable 2 for three different grids. The predicted methanol mole fraction distributions show that the deviations of the methanol mole frac-tion among these three grids are 1%. Therefore, the 91 26 grid is used in this work. The accuracy of the numerical results was vali-dated by comparing the predicted methanol conversion with

experimental results of Park et al.[5], and the agreement was good. More details were given elsewhere[16].

4. Results and discussion

The influences of the geometric parameters and thermo-fluid parameters on the performance of micro-reformer are considered of great importance. To this end, the effects of the channel length (L¼ 22 mm, 33 mm, and 44 mm), channel height (H ¼ 0.1 mm, 0.2 mm, and 1.0 mm), catalyst thickness (

d

2¼ 10

m

m, 30

m

m and

50

m

m) and catalyst porosity (

3

¼ 0.28, 0.38, and 0.48) on the methanol conversion and CO concentration in the micro-reformer were investigated. Additionally, the Reynolds number (Re_{¼ 2.2, 4.4} and 6.6), fuel ratio (

g

¼ 1.0, 1.3 and 1.6) and inlet temperature (T0¼ 100
C, 120
C, and 140
C) are the key thermo-fluid

param-eters in reformer channels which would affect the micro-reformer performance.

In Fig. 2(a), the effects of geometric parameters on methanol conversion of micro-reformer channel are presented as follows:

(a) The results show that the methanol conversion increases with an increase in the wall temperature Tw for all geometric 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.2 0.4 0.6 0.8 1 CH 3OH H 2O H 2 CO 2 CO

M

i

x/L

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.2 0.4 0.6 0.8 1 CH 3OH H 2O H 2 CO 2 CO

M

i

x/L

a

b

conditions, implying that a better micro-reformer performance can be archived at higher wall temperature. By comparing the results of Tw ¼ 200
C and Tw ¼ 260
C with L ¼ 33 mm,

H ¼ 0.2 mm,

d

2 ¼ 30

m

m and

3

¼ 0.38, it shows that the

methanol conversion for Tw¼ 260
C could be improved by

49%-points relative to that of Tw¼ 200
C.

(b) Comparison of the corresponding curves of the channel lengths of L ¼ 22 mm, 33 mm, and 44 mm indicates that better methanol conversion is found for a longer micro-reformer channel. This is due to the longer residence time of the fuel in the longer channel.

(c) As for the effects of channel heights (H_{¼ 0.1 mm, 0.2 mm, and} 1.0 mm), the results reveal that all the three micro-channels possess a similar methanol conversion.

(d) It is seen inFig. 2(a) that the methanol conversion in the micro-reformer is enhanced by the increased catalyst thickness. The channel with thicker catalyst layer has a larger chemical reac-tion area, which in turn causes a better methanol conversion. The results show that methanol conversion improves from 80% to 99% at Tw¼ 260
C with the catalyst thickness ranging from

10

m

m to 50

m

m.

(e) The effects of the porosity of catalyst layer (

3

¼ 0.28, 0.38, and 0.48) on methanol conversion of micro-reformer channel are also shown inFig. 2(a). It is found that the methanol conversion increases with an increase in the porosity of catalyst layer. This means that reaction surface is enlarged via an increase in the catalyst porosity.

The best methanol conversion is noted for the case with L¼ 33 mm, H ¼ 0.2 mm,

d

2¼ 50

m

m and

3

¼ 0.38 at Tw¼ 260
C.

This implies that the appropriate channel geometry and catalyst thickness are very critical for improving methanol conversion.

The CO concentration must be reduced for further use in a PEM fuel cell. Therefore, the CO concentration distributions for various geometric parameters are presented in Fig. 2(b). It is clearly observed that the CO concentration increases with increasing wall temperature. This is because the endothermic reverse wateregas-shift reaction increases as the wall temperature increases. It is clear in Fig. 2(b) that lower CO concentration is found for a micro-reformer with a shorter channel length. A detailed comparison of the corresponding curves shows that lower CO concentration is noted for a micro-reformer with a thinner catalyst thickness or

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.2 0.4 0.6 0. L=22mm, H=0.2mm, 2=30 m, =0.38 L=33mm, H=0.2mm, 2=30 m, =0.38 L=44mm, H=0.2mm, 2=30 m, =0.38 L=33mm, H=0.1mm, 2=30 m, =0.38 L=33mm, H=1.0mm, 2=30 m, =0.38 L=33mm, H=0.2mm, 2=10 m, =0.38 L=33mm, H=0.2mm, 2=50 m, =0.38 L=33mm, H=0.2mm, 2=30 m, =0.28 L=33mm, H=0.2mm, 2=30 m, =0.48

M

CH 3 OH

x/L

0 0.1 0.2 0.3 0.4 0.5 0.6 0 0.2 0.4 0.6 0. 8 1 8 1 L=22mm, H=0.2mm, 2=30 m, =0.38 L=33mm, H=0.2mm, 2=30 m, =0.38 L=44mm, H=0.2mm, 2=30 m, =0.38 L=33mm, H=0.1mm, 2=30 m, =0.38 L=33mm, H=1.0mm, 2=30 m, =0.38 L=33mm, H=0.2mm, 2=10 m, =0.38 L=33mm, H=0.2mm, 2=50 m, =0.38 L=33mm, H=0.2mm, 2=30 m, =0.28 L=33mm, H=0.2mm, 2=30 m, =0.48

M

CH 3 OH

x/L

a

b

a lower porosity. It is clearly seen that the CO concentration is about 16,000 ppm for the case with L¼ 33 mm, H ¼ 0.2 mm,

d

2¼ 30

m

m,

3

¼ 0.38 and a wall temperature of 260
_C.

The effects of inlet fuel ratio on the CO concentration (ppm) at the outlet were also investigated. The effects of the molar ratio of H2O/CH3OH on the CO concentration for various geometric

parameters and wall temperatures are shown inFig. 3. A careful inspection ofFig. 3discloses that the CO concentration decreases with an increase in the inlet molar ratio of H2O/CH3OH. This is due

to the fact that the higher H2O concentration enhances the

water-egas-shift reaction which, in turn, reduces the CO concentration. The results also show that the CO concentration would be reduced from 1.72% to 0.95% at Tw¼ 260
C with the H2O/CH3OH molar ratio

values ranging from 1.0 to 1.6. However, the higher molar ratio of H2O/CH3OH also reduces the H2 concentration at the channel

outlet. It is also found that the effects of the H2O/CH3OH molar ratio

on the CO concentration are more significant for a case with a higher wall temperature.

The impact of channel height on temperature distribution along the centerline of the channel, at afixed Reynolds number, was examined for the heights 0.1 mm, 0.2 mm and 1 cm. The

hydraulic diameters of channel vary depending on channel heights. A higher channel height has a greater hydraulic diameter. Fig. 4illustrates that the centerline temperature increases along the channel as a consequence of the heated wall. For a smaller channel height, the temperature distribution is much more uniform due to the shorter thermal entrance length. This kind of uniform temperature distribution improves the chemical reaction rate. Therefore, as shown in Fig. 2, the methanol conversion of the micro-reformer is slightly enhanced with the smaller channel height at higher wall temperature. A comparison of the temper-ature distributions for wall tempertemper-atures of 200
C and 260
C indicates that the centerline temperature increases with an increase in the wall temperature.

Fig. 5 shows the local distributions of the different species at wall temperatures of 200
C and 260
C along the centerline of the channel for the same operating conditions. Fig. 5 discloses that both the mole fractions of the CH3OH and H2O decrease as the

fluid moves downstream, while the H2, CO2and CO mole fractions

increase with axial location. Fig. 5clearly demonstrates that the mole fractions of the products increase with an increase in the wall temperature. In addition, Fig. 5(b) shows that the methanol

0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0. L=22mm, H=0.2mm, 2=30 m, =0.38 L=33mm, H=0.2mm, 2=30 m, =0.38 L=44mm, H=0.2mm, 2=30 m, =0.38 L=33mm, H=0.1mm, 2=30 m, =0.38 L=33mm, H=1.0mm, 2=30 m, =0.38 L=33mm, H=0.2mm, 2=10 m, =0.38 L=33mm, H=0.2mm, 2=50 m, =0.38 L=33mm, H=0.2mm, 2=30 m, =0.28 L=33mm, H=0.2mm, 2=30 m, =0.48

x/L

M H 2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.2 0.4 0.6 0. 8 1 8 1 L=22mm, H=0.2mm, 2=30 m, =0.38 L=33mm, H=0.2mm, 2=30 m, =0.38 L=44mm, H=0.2mm, 2=30 m, =0.38 L=33mm, H=0.1mm, 2=30 m, =0.38 L=33mm, H=1.0mm, 2=30 m, =0.38 L=33mm, H=0.2mm, 2=10 m, =0.38 L=33mm, H=0.2mm, 2=50 m, =0.38 L=33mm, H=0.2mm, 2=30 m, =0.28 L=33mm, H=0.2mm, 2=30 m, =0.48

x/L

M H 2

a

b

conversion is greater than 99% at a wall temperature of 260
C, with a product gas composition of 74.7% H2, 23.6% CO2and 1.7%

CO at the outlet of the channel. The results agree reasonably with experimental data[5]. For the PEM fuel cell, the CO concentration should be less than 10 ppm, so cleanup step is required after reforming. The utilization of the PrOx or wateregas-shift reaction equipment can reduce the CO concentration in the gas from the micro-reformer.

Studies of the reactant gas transport in micro-reformer chan-nels have shown that a detailed understanding of the local distribution of the CH3OH mole fraction along the channel is

important for designing the micro-reformer. Therefore, the effects of geometric parameters on the local distributions of the CH3OH

mole fraction along the channel centerline are presented inFig. 6. The results reveal that geometric parameters have a considerable impact on the local CH3OH distributions. It is found that the

CH3OH mole fractions decrease as thefluid moves downstream

due to the chemical reaction. For various channel heights, there appears to be little variation in the CH3OH mole fraction

distri-butions. The higher methanol concentration is noted for a system with a longer channel length or with a lower catalyst layer

thickness and porosity. This implies that the chemical reaction rate is weaker for a system with a shorter channel length or with a lower catalyst layer thickness and porosity. The effect of wall temperature on the local CH3OH mole fraction can be found by

comparing the corresponding curves inFig. 6(a) and (b). It is clear that smaller methanol concentration is noted for a case with a higher wall temperature. This can be explained by the fact that a stronger chemical reaction is experienced for a micro-reformer channel with a higher wall temperature.

The distributions of the H2mole fraction along the channel are

shown in Fig. 7 for various geometric parameters and wall temperatures. A higher H2mole fraction along the channel

repre-sents a higher methanol conversion. Thus, the variation of the H2

fraction is opposite to that of the CH3OH mole fraction inFig. 6. In

Fig. 7, a higher H2 mole fraction is found for a micro-reformer

channel with a longer channel length or with a higher catalyst thickness, porosity and wall temperature.

The Reynolds number increases with increasing fuel velocities at the same inlet area.Fig. 8 shows the effects of the Reynolds number on the local distributions of the CH3OH mole fraction

along the channel at the same catalyst layer thickness and

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.2 0.4 0.6 0. Re=2.2, =1.3, T 0=120 o C Re=4.4, =1.3, T 0=120 o_C Re=6.6, =1.3, T 0=120 o C Re=4.4, =1.0, T 0=120 o_C Re=4.4, =1.6, T 0=120 o C Re=4.4, =1.3, T 0=100 o_C Re=4.4, =1.3, T 0=140 o C

M

CH 3 OH

x/L

0 0.1 0.2 0.3 0.4 0.5 0.6 0 0.2 0.4 0.6 0. 8 1 8 1 Re=2.2, =1.3, T 0=120 o_C Re=4.4, =1.3, T 0=120 o C Re=6.6, =1.3, T 0=120 o_C Re=4.4, =1.0, T 0=120 o C Re=4.4, =1.6, T 0=120 o_C Re=4.4, =1.3, T 0=100 o C Re=4.4, =1.3, T 0=140 o_C

M

CH 3 OH

x/L

a

b

porosity. Three cases with Reynolds number of 2.2, 4.4 and 6.6 are presented in Fig. 8. It is clearly observed a lower methanol concentration (better methanol conversion) is found for a micro-reformer channel with a lower Reynolds number. This is due to the fact micro-reformer channel with a lower Reynolds number would experience a longer reactant gas resident time and reaction time, which in turn causes a better methanol conversion. In this work, we considered the H2O/CH3OH molar ratio values 1.0, 1.3

and 1.6. The predicted results in Fig. 8 show that the CH3OH

concentration decreases with an increase in the H2O/CH3OH

molar ratio. Also, the impact of different inlet fuel temperatures was examined for the values of 100
C, 120
C and 140
C. It is found that a lower CH3OH concentration (a better methanol

conversion) is found for a system with a higher inlet fuel temperature which increases the reaction rate. Comparison of the local CH3OH mole fractions in Fig. 8(a) and (b) for the wall

temperatures of 200
C and 260
C for the various operating parameters shows that the local CH3OH mole fraction decreases

with increasing wall temperature due to a strong chemical reac-tion for a high wall temperature.

The dependence of the local H2mole fraction distribution on

the Reynolds number, fuel ratio and inlet temperature are pre-sented inFig. 9. The results show that a higher H2mole fraction

is noted for a micro-reformer channel with a lower Reynolds number. This is due to the longer gas resident time which results in a better methanol conversion and a higher H2production. The

in_{fluences of the H}2O/CH3OH molar ratio on the H2 mole

frac-tion are presented inFig. 9. The results show that a higher molar ratio of H2O/CH3OH causes the H2 mole fraction to fall.

Addi-tionally, the H2mole fraction increases with increasing inlet fuel

temperature. Comparison of Fig. 9(a) and (b) indicates that a higher local H2 mole fraction is experienced for a

micro-reformer channel with a higher wall temperature owing to a stronger chemical reaction.

Fig. 10presents the variations of the CO mole fractions along the channel for the various thermo-fluid parameters. By comparing Figs. 9 and 10, it is found that the CO distributions inFig. 10have the same trends as the H2 distributions inFig. 9. This confirms the

common concept that a micro-reformer with a H2 production

indicates a higher CO concentration.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.2 0.4 0.6 0. Re=2.2, =1.3, T 0=120 o C Re=4.4, =1.3, T 0=120 o_C Re=6.6, =1.3, T 0=120 o C Re=4.4, =1.0, T 0=120 o C Re=4.4, =1.6, T 0=120 o C Re=4.4, =1.3, T 0=100 o C Re=4.4, =1.3, T 0=140 o C

x/L

M H 2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.2 0.4 0.6 0. 8 1 8 1 Re=2.2, =1.3, T 0=120 o C Re=4.4, =1.3, T 0=120 o_C Re=6.6, =1.3, T 0=120 o C Re=4.4, =1.0, T 0=120 o_C Re=4.4, =1.6, T 0=120 o C Re=4.4, =1.3, T 0=100 o_C Re=4.4, =1.3, T 0=140 o C

x/L

M H 2

a

b

5. Conclusions

This study numerically investigated the effects of the geometric and thermo-fluid parameters on the heat and mass transfer and methanol conversion in a micro-reformer channel. The conclusions drawn from the analyses are:

1. The effects of channel geometry and catalyst thickness have a significant impact on the methanol conversion and heat and mass transfer in the plate micro-reformer.

2. The micro-reformer performance increases with increasing wall temperature due to increase in the chemical reaction rate. 3. Smaller channel heights have much more uniform temperature distributions. Therefore, the methanol conversion of the micro-reformer is slightly enhanced with the smaller channel height at higher wall temperature.

4. A reduced Reynolds number for the reactant gas in the channel would rise the reactant gas residence time, which in turn, increases the reaction time and improves the methanol conversion.

5. The CO concentration decreases with increasing H2O/CH3OH

molar ratio of the inlet fuel.

Acknowledgement

This study was supported by the National Science Council through grant number NSC 96-2212-E-211-004.

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0 100 5 10-5 1 10-4 1.5 10-4 2 10-4 2.5 10-4 3 10-4 3.5 10-4 4 10-4 0 0.2 0.4 0.6 0. Re=2.2, =1.3, T 0=120 o C Re=4.4, =1.3, T 0=120 o_C Re=6.6, =1.3, T 0=120 o C Re=4.4, =1.0, T 0=120 o_C Re=4.4, =1.6, T 0=120 o C Re=4.4, =1.3, T 0=100 o C Re=4.4, =1.3, T 0=140 o C

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