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Three-dimensional analysis of a plate methanol steam micro-reformer and a methanol catalytic combustor with different flow channel designs
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 3 5 7 5 e1 3 5 8 6
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Three-dimensional analysis of a plate methanol steam microreformer and a methanol catalytic combustor with different flow channel designs Ching-Yi Hsueh a, Hsin-Sen Chu b, Wei-Mon Yan c,*, Guang-Ching Leu a, Jong-Ian Tsai a a
Network Operations Laboratory, Chunghwa Telecommunication Laborateries, Yang-Mei, Tao-Yuan 326, Taiwan, ROC Industrial Technology Research Institute, Chu-Tung, Hsin-Chu 310, Taiwan, ROC c Department of Greenergy, National University of Tainan, Tainan 700, Taiwan, ROC b
article info
abstract
Article history:
Three-dimensional models of a plate methanol steam micro-reformer and a methanol
Received 30 March 2011
catalytic combustor with parallel flow fields and serpentine flow fields have been estab-
Received in revised form
lished. The effects of the flow field design and the fuel flow rate on the methanol
7 July 2011
conversion and transport phenomena in the micro-reformer were investigated. The results
Accepted 23 July 2011
revealed that the methanol conversion of the micro-reformer with the serpentine flow field and the combustor with the serpentine flow field has been optimized as a result of improved thermal management in the micro-reformer with combustor. With a change in
Keywords:
flow field design from the micro-reformer and the combustor with parallel flow fields to the
Micro-reformer
micro-reformer and combustor with the serpentine flow fields a wall temperature increase
Catalytic combustor
from 225 C to 237 C was observed. The methanol conversion of the micro-reformer with
Methanol
the serpentine flow field and the combustor with the serpentine flow field could be
Flow field designs
improved by 23% relative to the employment of a parallel flow field. A numerical model provided an efficient way to characterize the transport phenomena within the microreformer and combustor; the results will benefit the future design of plate methanol steam micro-reformers with combustors. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
A methanol steam micro-reformer is a chemical device that drives the conversion of methanol to hydrogen. In a proton exchange membrane fuel cell (PEMFC), electricity and water are produced from the combination of hydrogen and oxygen in an electrochemical reaction. A small PEMFC with methanol steam micro-reformer can hold a high energy density, while operating with a low noise and pollution output; this makes it a potential candidate for portable electronic products in the near future. Therefore, the vast literature devoted to the
experiments for the methanol steam micro-reformer has been reviewed on several occasions [1e5]. A number of studies have been done on the transport phenomena and performance of the micro-reformer. In order to simplify the analysis, many studies only considered the plate micro-reformer, namely the catalytic combustor is not included in the analysis [6e10]. Hsueh and collaborators [6e8] developed a mathematical model for the plate methanol steam micro-reformer to investigate the effects of geometric and thermo-fluid parameters on performance as well as heat and mass transfer phenomena in micro-reformer channels.
* Corresponding author. Tel.: þ886 6 260 2251; fax: þ886 6 260 2205. E-mail address: [email protected] (W.-M. Yan). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.07.099
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RMD
Nomenclature Ci cp D Deff Dk Dp Ea H HB , HT HC HR HW DH I, J, K keff kf kp ks k1 k2 k3 k4 L Mi Mw,i p R RSR RrWSG
3
concentration of species i, mol m specific heat at constant pressure, J kg1 K1 hydraulic diameter, m effective mass diffusivity, m2 s1 mass diffusion coefficient, m2 s1 catalyst particle diameter (m) activation energy, J mol1 micro-reformer height, m steel height, m combustion flow channel, m reforming flow channel, m solid wall thickness, m Enthalpy of reaction, J mol1 grid points in the x, y and z directions, respectively effective thermal conductivity, W m1 K1 fluid phase thermal conductivity, W m1 K1 permeability, m2 solid medium thermal conductivity, W m1 K1 pre-exponential factor for steam reforming pre-exponential factor for the reverse water gas shift pre-exponential factor for decomposition reaction pre-exponential factor for combustion reaction flow channel length, m mole fraction of species i molecular weight of species i, kg mol1 pressure, Pa universal gas constant Arrhenius reaction rate coefficient for steam reforming, mol m3 s1 Arrhenius reaction rate coefficient for the reverse water gas shift, mol m3 s1
The tendencies of parameters (including longer channel length, smaller channel height, thicker catalyst layer, smaller aspect ratio, larger catalyst porosity, lower Reynolds number and higher wall temperature) to enhance the performance of the micro-reformer were identified. A model of the plate methane reformer to explore the temperature and gas distributions in the channels was developed by Yuan et al. [9,10]. The results indicated that small thermal conductivity of the catalyst layer and the solid walls have high reforming reaction rates. The effects of heat and mass transfer characteristics in a cylindrical mathematical model of a packed bed reformer are presented by several investigators [11e14]. Suh et al. [11,12] found that the internally heated reformer could improve methanol conversion. Karim et al. [13,14] showed the minimum reformer diameter had the highest catalyst activity and smallest temperature gradient. The theoretical modeling of steam reforming coupled with catalytic combustion for the plate reactors has been studied by several investigators [15e19]. The catalytic combustor supplied heat to the steam reforming reaction, and hydrogen was produced by the micro methanol steam reformer. A onedimensional transient mathematical model to study the transport phenomena in a methanol steam reformer with
Arrhenius reaction rate coefficient for decomposition reaction, mol m3 s1 RCombustion Arrhenius reaction rate coefficient for combustion reaction, mol m3 s1 T temperature, C inlet temperature, C T0 wall temperature, C Tw u, v, w velocity components in the x, y and z directions, respectively, m s1 Y Y ¼ y/HC þ dC þ HW þ dR þ HR x, y, z coordinates, m W micro-reformer width, m steel width, m W1 channel width, m W2 rib width, m W3 Greek symbols combustion catalyst layer thickness, m dC Reforming catalyst layer thickness, m dR ε porosity h methanol conversion s tortuousity of the porous medium m viscosity, kg m1 s1 r density, kg m3 rs catalyst density, kg m3 Subscripts eff effective u x-direction v y-direction w z-direction 0 inlet
a combustor was presented by Varesano et al. [15]. The transient characteristics of the reformer were examined in detail. Hsueh et al. [16] developed a three-dimensional numerical model of a micro-reformer with combustor to examine the effects of various flow configurations and the flow channel on the performance of the micro-reformer. Comparing the coand counter-current flows via numerical simulation, their results show that methanol conversion for counter-current flow could be improved by 10%. The results also showed that an improved micro-reformer performance was given by a lower reforming channel height. Arzamendi et al. [17,18] established a micro-channel model using thermal integration of a steam reformer and a catalytic combustor. Using the hydrogen produced by the reforming reaction from methanol and methane, the results showed the short diffusion distance and higher area to volume ratio required for using the micro reactor. The results also indicated that complete combustion takes place over a very short distance. The reforming fuel is rapidly heated, subsequently giving the reactor a more uniform temperature distribution. Pan and Wang [19] developed a plate-fin reformer which integrated endothermic and exothermic reactions into one unit. The combustor supplied the heat for the methanol steam reformer. Numerical
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simulation results accurately predicted the methanol conversion rate and gas distributions. They also indicated that the temperature distributions were much more uniform in the plate-fin reformer. In order to simplify the analysis, Varesano et al. [15] has considered the numerical model of methanol steam reformers, only including energy equation and concentration equations with chemical reaction. Furthermore, the continuity equation, momentum equation, energy equation and species equations with chemical reaction were employed in the reformer with the combustor by several researchers [16e19]. However, they have studied plate steam reformers with a parallel flow field which is attractive due to its simplicity. In this study, an attempt is made to examine the detailed fluid flow, heat and mass transfer coupled with chemical reactions in a three-dimensional computational model of the plate methanol micro-reformer with a methanol catalytic combustor for the serpentine and the parallel flow field.
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The flow field design in a fuel cell is one of the most important issues for a PEMFC. An appropriate flow field design in the fuel cell can improve the reactant transport, as well as thermal and water management. To this end, different flow field configurations, including parallel, serpentine and interdigitated have been developed. Many efforts have been devoted to optimize the flow field design to improve cell performance [20e23]. In recent years, several studies based on flow field design theory were applied to the plate methanol microreformer. Jang et al. [24] developed five inlet and outlet manifold configurations in a three-dimensional numerical model. The results showed how the configuration of a central inlet with two outlets could improve the methanol conversion to 23%. A plate methanol steam micro-reformer model with a serpentine flow field was built and its transport phenomena and performance was investigated and presented by Hsueh et al. [25]. Top plate heating with the serpentine flow field had a higher methanol conversion than with bottom plate heating.
a
L
HT HC C
y
H HW
x
Y=0.333
R
Y=0.167
HR 0
HB W1
b
z W2
W3
W
c
Fig. 1 e (a) Schematic diagram of a plate methanol steam micro-reformer with methanol catalytic combustor, (b) parallel flow field and (c) serpentine flow field.
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Different types of flow field designs for plate methanol microreformers have been used to achieve more efficient methanol conversion. Kundu et al. [26] used different flow configurations, including serpentine and parallel flow fields, to improve plate methanol reformer performance. The results showed the serpentine flow field had a better durability than the parallel flow field. From the literature cited above, it was shown that methanol conversion can be enhanced by a suitable flow channel design. However, only limited work has been done to investigate the effect of different flow field designs on the performance of the plate methanol micro-reformer with methanol catalytic combustor. Therefore, the objective of this paper is to establish a three-dimensional computational model of the plate methanol micro-reformer with a methanol catalytic combustor to investigate the performance and transport phenomena of the micro-reformer with various flow fields (parallel flow field and serpentine flow field).
2.
Analysis
2 vv vv vv vp v v v2 v v2 v εr u þ v þ w þ 2 þ 2 þ Sv ¼ ε þ εm 2 vx vy vz vy vx vy vz
(3)
Z-momentum equation: 2 vw vw vw vp v w v2 w v2 w εr u þ v þ w þ Sw þ þ ¼ ε þ εm vx vy vz vz vx2 vy2 vz2
(4)
In the above equations, ε is the porosity of the medium, and the mixture viscosity mmix is [27] 5 X
mmix ¼
i¼1
Mi mi P5 j¼1 Mj fij
(5)
where 2
fij ¼
In this study, three-dimensional computational models with various flow fields have been established for a methanol steam micro-reformer with a methanol catalytic combustor. The flow fields in the methanol steam micro-reformer and methanol catalytic combustor include the parallel flow field and the serpentine flow field. The parallel flow field has five parallel channels and the serpentine flow field has one channel with four turns. A schematic illustration of these flow fields and associated coordinate system are shown in Fig. 1. The parallel flow field has five flow channels and each channel is 40 mm in length. The serpentine flow field has one flow channel, the total flow channel length is five times the length of a channel in the parallel flow field, and there are four turning points. In this study, a constant flow rate approach is utilized to investigate the effect of the flow field on the performance of the microreformer. The letters u, v, and w are the velocity components in the x-, y-, and z-directions, respectively. The reactor consists of a solid wall, a steam reforming flow channel, a steam reforming catalyst layer, a catalytic combustion catalyst layer and a catalytic combustion flow channel. To simplify the analysis, it is assumed that, the flow is in a steady state, the inlet fuel is an ideal gas, the flow is laminar and incompressible, the catalyst layer is isotropic, the chemical reaction occurs only in the catalyst layer and thermal radiation and conduction in the gas phase are negligible compared to convection. The transport equations for the three-dimensional model are given as:
X
6 41 þ
32 !1 1 mi 2 Mw;j 4 7 5 mj Mw;i
1 Mw;i 2 8 1þ Mw;j
i
(6)
In the momentum equations, Su, Sv and Sw are correction terms of the reactant gas flow in a porous material in the catalyst layer. In this work, the flow channel is not a porous medium, therefore, Su, Sv and Sw are zero in the flow channel. However, the catalyst layer is a porous medium, the source terms, Su, Sv and Sw account for the Ergun equations [28] in the x-, y- and z-directions, respectively. The parameter kp is the permeability and b is the inertial loss coefficient in each component direction. ffi mu burpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u2 þ v2 þ w2 Su ¼ kp 2
(7)
ffi mv bvrpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Sv ¼ u2 þ v2 þ w2 kp 2
(8)
ffi mw bwrpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Sw ¼ u2 þ v2 þ w2 kp 2
(9)
where kp ¼
b¼
D2p ε3 150ð1 εÞ2
3:5ð1 εÞ Dp ε3
(10)
(11)
and where Dp is the catalyst particle diameter. Species equation:
Continuity equation: vu vv vw þ þ ¼0 r vx vy vz
Y-momentum equation:
(1)
2 vmi vmi vmi v mi v2 mi v2 mi u þv þw ¼Deff þ 2 þ 2 2 vx vy vz vx vy vz
(12)
þð1εÞrs Sc X-momentum equation:
2 vu vu vu vp v u v2 u v2 u εr u þ v þ w þ 2 þ 2 þ Su ¼ ε þ εm 2 vx vy vz vx vx vy vz
(2)
In the species equation, mi denotes the mass fraction of the ith species, where the various species are CH3OH, H2O, H2, CO2, CO and O2. In these expressions, the concentrations of CH3OH, H2O, H2, CO2, CO are calculated on the steam reforming side and CH3OH, H2O, CO2, O2 are calculated on the
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combustion side. Deff is the effective diffusion coefficient based on the StefaneMaxwell equations [27]. Eq. (13) is employed to describe the influence of the porosity on the diffusion coefficient. Deff ¼ Dk εs
(13)
The diffusion coefficient Dk for the methanol steam microreformer was derived from the StefaneMaxwell equations which were used to calculate the mean effective binary diffusivity [17]. Sc represents the source term due to the chemical reaction in the catalyst layer. In this study, there was no chemical reaction in the flow channel. Therefore, Sc was zero in the flow channel. In the catalyst layer, Sc differs according to the chemical reactions in the catalyst layer. The source term of the species equation, Sc, can be described by the following equations: Mw;i ðRSR þ RrWGS þ RMD Þ l00i l0j for steam reforming Sc ¼ Mw;i ðRCombustion Þ l00i l0j for combustion reaction
(14)
where Mw,i is the molecular weight of species i, RSR, RrWGS, RMD and RCombustion are the Arrhenius molar rates in the reaction, l}i and lj are the stoichiometric coefficients for reaction i and product j, respectively, in the reaction. According to the chemical kinetics of Pepply et al. [29], the methanol steam reforming reaction consists of three overall reactions: one is the primary process in the methanol steam reforming and the others are the decomposition reaction and the wateregas shift reaction. Therefore, the steam reforming reaction, Eq. (15), the reverse wateregas shift reaction, Eq. (16), and the decomposition reaction, Eq. (17), are each considered in the present study. k1
CH3 OH þ H2 O ! CO2 þ 3H2 k1
k2
CO2 þ H2 ! CO þ H2 O k2
k3
CH3 OH/CO þ 2H2
(15)
(16)
(17)
In this study, to simplify the analysis, RSR, RrWGS and RMD are the methanol steam reforming reaction bases on the Mastalir et al. [30]. The Arrhenius equation is employed to calculate chemical reaction rate coefficients: Ea Ea 0:4 k C exp C C exp RSR ¼ k1 C0:6 1 CO H CH3 OH H2 O 2 2 RT RT
(18)
Ea Ea RrWGS ¼ k2 CCO2 CH2 exp k2 CCO2 CH2 O exp RT RT
(19)
Ea RMD ¼ k3 C1:3 exp CH3 OH RT
(20)
where the steam reforming reaction and reverse wateregas shift reaction are reversible reactions and the decomposition reaction is a non-reversible reaction. The constants k1, k2 and k3 are the forward rate constants, and the constant k1 and k2 are the backward rate constants.
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The reaction of the catalytic combustion catalyst layer can be represented by Eq. (21). The reaction rate coefficient of the methanol combustion reaction over the Pt/Al2O3 catalyst was calculated with Eq. (22), as proposed by Pasel et al. [31] k4
CH3 OH þ 1:5O2 /CO2 þ 2H2 O
(21)
Ea exp RCombustion ¼ k4 C1:3 CH3 OH RT
(22)
To evaluate the local temperature distributions, the energy equations must be solved. Energy equation: 2 vT vT vT v T v2 T v2 T þ ð1 εÞrs St þ þ ¼ keff rcp u þ v þ w vx vy vz vx2 vy2 vz2
(23)
keff is the modified effective thermal conductivity of the porous medium which is given by keff ¼ εkf þ ð1 εÞks
(24)
where kf is the fluid phase thermal conductivity, ks is the solid medium thermal conductivity and ε is the porosity of the medium. St in the channel is zero. The catalyst layer experiences exothermic and endothermic chemical reactions, so the source term St in the energy equation due to the chemical reactions is determined by
St ¼
ðDHSR RSR þDHrWGS RrWGS þDHMD RMD Þ forsteamreforming ðDHCombustion RCombustion Þ forcombustionreaction (25)
The energy equation of the solid wall can be calculated by v2 T v2 T v2 T þ þ ¼0 vx2 vy2 vz2
(26)
The boundary conditions at the inlet, outlet, wall, and the interface between the flow channel and the catalyst layer were as follows: the inlet flow velocity is constant, the inlet gas composition is constant and the inlet temperature is constant. At the flow channel and the catalyst layer, there is fully developed flow. At the interface between the solid wall and the insulated walls, the temperature gradients were zero. At the interface between the flow channel and the solid wall, no slip and zero fluxes held the velocities and the concentration gradients at zero. For the interface between the flow channel and the catalyst layer, the velocities, temperatures, species concentrations and species fluxes are continuous. The governing equations were made discrete by employing a finite volume scheme and subsequently solved using the commercial fluid dynamics program Fluent. Details of the numerical solutions have been given in previous studies [16]. The convergence criteria for the normalized residuals for each variable were restricted to less than 106. The dimensions of the plate methanol steam micro-reformer with a methanol catalytic combustor were 40 mm (L) 15 mm (W ) 2.5 mm (H ), the inlet and outlet cross-sections of the flow channel were 1.0 mm 0.5 mm, and the rib width (W3) was 2 mm. The thickness of the catalyst layer was set to 0.05 mm. The operating pressure and temperature of both micro-reformer and combustor were 1 atm and 393 C, respectively, the inlet flow
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320
Table 1 e Parameters used in this study.
300 280
o
260
w
4 102 5.0 105 5.0 105 4.5 104 4.5 104 1 103 1 103 2 103 120 1 1480 0.3
T ( C)
Length L (m) Combustion catalyst layer thickness dC (m) Reforming catalyst layer thickness dR (m) Combustion flow channel HC (m) Reforming flow channel HR (m) Steel width W1 (m) Channel width W2 (m) Rib width W3 (m) Average inlet temperature ( C) Operating pressure (atm) Catalyst density (kg m3) [13] Catalyst thermal conductivity (W m1 k1) [13] Catalyst layer porosity [33] Catalyst permeability (m2) [33] Mass diffusion coefficient (m2 s1) [17] Activation energy for steam reforming (J mol1) [30] Activation energy for the reverse water gas shift (J mol1) [30] Activation energy for decomposition reaction (J mol1) [30] Activation energy for combustion reaction (J mol1) [31] Pre-exponential factor for steam reforming Pre-exponential factor for the reverse water gas shift Pre-exponential factor for decomposition reaction Pre-exponential factor for combustion reaction
240 220
180
0.3
0.35
0.4
u
0.38 2.379 1012 6.8 105
0.45
-1
0,C
(m s )
100
1.09 105
80
1.15 105
60
1.42 10
Present Results Experimental Study of Won et al. [32]
200
5
40
1.3 104 20
Present Results Experimental Study of Won et al. [32]
9.55 1012 0 200
1.65 1013
220
240
260
280
300
o
T ( C) w
1.65 1013 5.8 105
rates at the reforming side and at the combustion side were 7.5 cm3 min1 and 75 cm3 min1 respectively. The parallel flow field and the serpentine flow field have the same inlet flow rates. The geometrical dimensions and parameters used are listed in Table 1. In order to simplify the analysis, three grid systems were evaluated for the single channel of the plate methanol steam micro-reformer with combustor. The grid numbers in the x-, yand z-directions were 151 71 10, 161 76 13 and 171 81 16, respectively. To examine the independence of the grid predictions, three grid systems were considered and their influences on the prediction of local temperature distributions for a typical case are presented in Table 2. It was found that the maximum deviation among the computations using grids of 151 71 10, 161 76 13 and 171 81 16 was less than 0.76% and the results on the 161 76 13 and the 171 81 16 grids were relatively close. Therefore, the parallel flow field grid 161 76 61 was chosen for the simulation model in making a tradeoff between accuracy and CPU time.
Fig. 2 e Comparison of theoretical simulation of present results with previous experimental data by Won et al. [32].
Fig. 2 gives a comparison of calculated methanol conversion and wall temperature with experimental data [32]. Only a small discrepancy was found between the numerical results and the experimental data. Preliminary runs confirmed that the micro-reformer performance and gas transport phenomena could be accurately predicted using our simulation. This model has the potential to reduce the design time of a plate methanol steam micro-reformer with combustor.
3.
Results and discussion
This paper aimed to establish three-dimensional computational models of a plate methanol steam micro-reformer with a methanol catalytic combustor to investigate the performance and transport phenomena of parallel and serpentine flow fields. Due to discrepancies in the flow channel design, flow channel length and the turning points among various flow field designs; the fuel consumption and temperature
Table 2 e Temperature distributions ( C) for the various grid tests at different axial locations. X IJK 151 71 10 161 76 13 171 81 16
0.125 224.9 224.0 223.2
0.250 225.6 224.7 223.9
0.375 225.9 225.1 224.3
0.500 226.1 225.3 224.5
0.625 226.2 225.4 224.6
0.750 226.2 225.4 224.6
0.875 226.3 225.4 224.6
1.000 226.3 225.4 224.6
Fig. 3 e The velocity distributions on the middle cross-section of the reforming channel (Y [ 0.167) for reformer and combustor with various flow field designs.
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distributions inside the micro-reformer with combustor were found to be different. The effects of the flow field designs on the velocity, temperature, CH3OH mole fraction, H2 mole fraction and CO mole fraction distributions were investigated and are presented in this study. The velocity distribution on the middle cross-section of the reforming channel (Y ¼ 0.167) is presented in Fig.3. In the serpentine flow field, the flow channel values were found to be one-fifth of that of the parallel flow field. The fuel velocity increases due to a greater channel convection effect. It can be seen that recirculation flow takes place at the turning points of the flow channels due to the change of velocity direction, generating shear stresses, which increase fuel consumption. The temperature distributions on the top cross-section of the reforming channel (Y ¼ 0.333) are presented in Fig. 4. A constant flow rate was employed in this analysis. The temperature distribution along the channel was seen to increase as a consequence of the chemical reaction along the flow channels. For the combustor, the results indicated a higher temperature for the combustor with serpentine flow field. This can be made plausible by noting the fact that the serpentine flow field had a higher fuel flow velocity than the parallel flow field, and that the fuel experiences several turning points before reaching the outlet; this increases the energy released. In regards to the reformer, a higher temperature distribution was noted in the serpentine flow field. This may be because the smaller outlet area decreased the heat leaving the flow channel. Therefore, the serpentine flow field provided better thermal management than a parallel flow field. The combustor with serpentine flow field and the microreformer with serpentine flow field have the highest temperature distribution for each of the channel designs. Fig. 5 presents the CH3OH mole fraction distributions on the middle cross-section of the reforming channel (Y ¼ 0.167) for the various flow fields. It shows that the CH3OH mole fraction decreased along the channel for the four channel designs due to the methanol steam reforming reaction. By comparing the CH3OH mole fraction distributions, the highest methanol concentration was noted for the combustor with a parallel flow field and the micro-reformer with a parallel flow field. This implies that the chemical reaction rate was slowest for the combustor with a parallel flow field and the micro-reformer with a parallel flow field due their having the lowest temperature distribution. The methanol conversion rates at the channel exits were; 52%, 62%, 68%, and 79% for Fig. 5(a) the combustor with parallel flow field and the microreformer with parallel flow field, Fig. 5(c) the serpentine flow field with combustor and the micro-reformer with parallel flow field, Fig. 5(b) the combustor with parallel flow field and the micro-reformer with serpentine flow field, Fig. 5(d) the combustor with serpentine flow field and the micro-reformer with serpentine flow field, respectively. Therefore, it was expected that the methanol conversion would be highest for the combustor with a serpentine flow field and the microreformer with a serpentine flow field. The distributions of the H2 mole fraction on the middle cross-section of the reforming channel (Y ¼ 0.167) are shown in Fig. 6 for the various flow fields. A higher H2 mole fraction along the flow channel represents greater methanol conversion. Thus, the variation of the H2 fraction was opposite to
Fig. 4 e The temperature distributions on the top crosssection of the reforming channel (Y [ 0.333) for reformer and combustor with various flow field designs.
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Fig. 5 e The CH3OH mole fraction distributions on the middle cross-section of the reforming channel (Y [ 0.167) for reformer and combustor with various flow field designs.
Fig. 6 e The H2 mole fraction distributions on the middle cross-section of the reforming channel (Y [ 0.167) for reformer and combustor with various flow field designs.
that of the CH3OH mole fraction. For the combustor, the serpentine flow field had a greater temperature distribution than the parallel flow field. In regards to the reformer, the serpentine flow field had a greater chemical reactivity than
the parallel flow field due since the fuel remained in the channel longer and thus the serpentine flow field provided a better methanol conversion. Therefore, the highest H2 mole fraction was found for the combustor with a serpentine flow
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confirmed the common belief that a micro-reformer with H2 production indicates a higher CO concentration. For the combustor with serpentine flow field and the micro-reformer with a serpentine flow field, the CO mole fraction was 746 ppm at the outlet of the reforming channel. The inlet flow rate (Q0,R) on the micro-reformer side is a significant issue to be considered when choosing the flow field designs. Fig. 8(a) shows the effects of the inlet flow rate (Q0,R) on the micro-reformer side on the wall temperature of the reforming channel for various flow field designs. The results revealed that the wall temperature was increased by the decreased inlet flow rate (Q0,R) on the micro-reformer. This was due to a higher inlet flow rate (Q0,R) significantly increasing the heat leaving the flow channel, which reduced the rise in temperature. The combustor with the serpentine flow field and the micro-reformer with the serpentine flow field had the highest wall temperature of the reforming channel of all other flow fields. In addition, it is seen in Fig. 8(b) that a higher methanol conversion (h) increases with a lower inlet flow rate (Q0,R) due to a longer gas residence time and a higher temperature. The methanol conversion of the serpentine flow field with combustor and the serpentine flow field with micro-reformer was the best due to a higher wall temperature. The effects of the flow field designs on the inlet flow rate (Q0,C) of the combustor and wall temperature of the reforming channel are presented in Fig. 9. It indicates that the
a
300 Combustor (Parallel Flow Field), Reformer (Parallel Flow Field) Combustor (Parallel Flow Field), Reformer (Serpentine Flow Field) Combustor (Serpentine Flow Field), Reformer (Parallel Flow Field) Combustor (Serpentine Flow Field), Reformer (Serpentine Flow Field)
260
w
o
T ( C)
280
240
220
200 2
4
6
Q
b
8 3
0,R
10
12
-1
(cm min )
100 (b) 80
(%)
60
40
Fig. 7 e The CO mole fraction distributions on the middle cross-section of the reforming channel (Y [ 0.167) for reformer and combustor with various flow field designs.
Combustor (Parallel Flow Field), Reformer (Parallel Flow Field) Combustor (Parallel Flow Field), Reformer (Serpentine Flow Field) Combustor (Serpentine Flow Field), Reformer (Parallel Flow Field) Combustor (Serpentine Flow Field), Reformer (Serpentine Flow Field)
20
0 2
field and the micro-reformer with a serpentine flow field. Fig. 7 presents the effects of the various flow field designs on the CO mole fraction distributions on the middle cross-section of the reforming channel (Y ¼ 0.167). It was found that the CO distributions had the same trends as the H2 distributions. This
4
6
8
Q
(cm min )
3
0,R
10
12
-1
Fig. 8 e Effects of the inlet flow rate (Q0,R) of the reformer and various flow field designs on (a) wall temperature and (b) methanol conversion.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 3 5 7 5 e1 3 5 8 6
a
260
220
w
o
T ( C)
240
200 Combustor (Parallel Flow Field), Reformer (Parallel Flow Field) Combustor (Parallel Flow Field), Reformer (Serpentine Flow Field)
180
Combustor (Serpentine Flow Field), Reformer (Parallel Flow Field) Combustor (Serpentine Flow Field), Reformer (Serpentine Flow Field)
160 20
40
60
Q
b
80 3
0,C
100
120
-1
(cm min )
100
80
60
40
Combustor (Parallel Flow Field), Reformer (Parallel Flow Field) Combustor (Parallel Flow Field), Reformer (Serpentine Flow Field) Combustor (Serpentine Flow Field), Reformer (Parallel Flow Field) Combustor (Serpentine Flow Field), Reformer (Serpentine Flow Field)
20
0 20
40
60
Q
80 3
0,C
100
120
-1
(cm min )
Fig. 9 e Effects of inlet flow rate (Q0,C) of the combustor and various flow field designs on (a) wall temperature and (b) methanol conversion.
temperature increased with an increase in the inlet flow rate (Q0,C) on the combustor side due to a higher inlet flow rate. This is because greater combustion energy was released due to a higher inlet flow rate. The wall temperature was observed to increase in the following order: the combustor with parallel flow field and the micro-reformer with parallel flow field; the combustor with serpentine flow field and the micro-reformer with parallel flow field; the combustor with parallel flow field
10
6
H2
3
-1
Q (cm min )
8
4 Combustor (Parallel Flow Field), Reformer (Parallel Flow Field) Combustor (Parallel Flow Field), Reformer (Serpentine Flow Field) Combustor (Serpentine Flow Field), Reformer (Parallel Flow Field) Combustor (Serpentine Flow Field), Reformer (Serpentine Flow Field)
2
0 2
4
6
Q
3
0,R
8
-1
10
12
(cm min )
Fig. 10 e Effects of the inlet flow rate (Q0,R) of the reformer and various flow field designs on the H2 production rate.
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and the micro-reformer with serpentine flow field; the combustor with serpentine flow field and the micro-reformer with serpentine flow field. This was due to improvements in heat transport within the flow field design. In addition, the effects of the flow field designs on the inlet flow rate (Q0,C) of the combustor and methanol conversion in the plate methanol steam micro-reformer are shown in Fig. 9(b). In regards to methanol conversion, the results indicate that the methanol conversion increased as the inlet flow rate (Q0,C) of the combustion channel increased. The highest efficiency methanol conversion was enhanced by the combustor with serpentine flow field and the micro-reformer with serpentine flow field. Therefore, it is concluded that increasing the number of corners and channel length to the various flow fields can effectively raise the temperature distribution and thereby enhance the methanol conversion. Fig. 10 demonstrates the effects of the inlet flow rate (Q0,R) on the micro-reformer side on the H2 production rate of the channel for various flow fields. Fig. 10 implies that the H2 production rate (QH2) increased with an increasing inlet flow rate (Q0,R) on the micro-reformer side due to a higher inlet flow rate. By comparing Figs. 8 and 10, QH2 increased and h decreased with an increase in Q0,R. Therefore, it is important to note that a higher inlet flow rate (Q0,R) on the microreformer side will not necessarily provide a better H2 production rate. When the methanol conversion was too small, a higher inlet flow rate (Q0,R) provided a lower H2 production rate. The combustor with serpentine flow field and the micro-reformer with serpentine flow field was the best design when considering the H2 production rate. When the combustor with serpentine flow field and the micro-reformer with serpentine flow field for a Q0,R of 9 cm3 min1, the methanol conversion was 67%, and the H2 production rate was 8 cm3 min1.
4.
Conclusions
Three-dimensional numerical models of a methanol steam micro-reformer with a methanol catalytic combustion chamber have been developed in this analysis. The flow fields including the parallel flow field and the serpentine flow fields were investigated. The temperature, methanol mole fraction, hydrogen mole fraction and CO mole fraction for the various flow fields have been studied in detail. 1. A higher temperature distribution was found for the plate methanol steam micro-reformer with the serpentine flow field as well as the methanol catalytic combustor with the serpentine flow field. 2. The plate methanol steam micro-reformer with the serpentine flow field and the methanol catalytic combustor with the serpentine flow field were found to have a better methanol conversion than any other flow field design in this study. 3. With a change in flow field design from the micro-reformer and the combustor with parallel flow fields to the microreformer and combustor with the serpentine flow fields a wall temperature increase from 225 C to 237 C was observed.
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4 .The methanol conversion for the micro-reformer with the serpentine flow field and the combustor with the serpentine flow field could be improved by 23% relative to that of the micro-reformer with the parallel flow field and the combustor with the parallel flow field.
Acknowledgment The study was supported by the National Science Council, the Republic of China, through the grants NSC 97-2221-E-211-015MY2.
references
[1] Kim T. Micro methanol reformer combined with a catalytic combustor for a PEM fuel cell. Int J Hydrogen Energy 2009; 34(16):6790e8. [2] Lee CY, Lee SJ, Shen CC, Chang CC, Lin CH, Yeh CT. Microsensors with novel micro-channels in a micro-reformer. Int J Hydrogen Energy 2010;35(19):10630e4. [3] Jin J, Kwon S. Fabrication and performance test of catalytic micro-combustors as a heat source of methanol steam reformer. Int J Hydrogen Energy 2010;35(4):1803e11. [4] Pan L, Ni C, Zhang X, Yuan Z, Zhang C, Wang S. Study on a compact methanol reformer for a miniature fuel cell. Int J Hydrogen Energy 2011;36(1):319e25. [5] Lee CY, Lee SJ, Shen CC, Yeh CT, Chang CC, Chang YM. Insitu measurement of the local temperature distributions for the steam reforming of a methanol micro reformer by using flexible micro temperature sensors. Int J Hydrogen Energy 2011;36(4):2869e76. [6] Hsueh CY, Chu HS, Yan WM. Numerical study on microreformer performance and local transport phenomena of the plate methanol steam micro-reformer. J Power Sources 2009; 187(2):535e43. [7] Chen F, Chang MH, Kuo CY, Hsueh CY, Yan WM. Analysis of a plate-type microreformer for methanol steam reforming reaction. Energy Fuels 2009;23(10):5092e8. [8] Hsueh CY, Chu HS, Yan WM, Chen CH, Chang MH. Numerical study of heat and mass transfer in the plate methanol steam micro-reformer channels. Appl Therm Eng 2010;30(11e12): 1426e37. [9] Yuan J, Ren F, Sunden B. Analysis of chemical-reactioncoupled mass and heat transport phenomena in a methane reformer duct for PEMFCs. Int J Heat Mass Transfer 2007; 50(3e4):687e701. [10] Yuan J, Yang G, Sunden B. On reaction coupled transport phenomenon in reformer ducts. Int J Hydrogen Energy 2010; 35(13):7183e8. [11] Suh JS, Lee MT, Greif R, Grigoropoulos CP. A study of steam methanol reforming in a microreactor. J Power Sources 2007; 173(1):458e66. [12] Suh JS, Lee MT, Greif R, Grigoropoulos CP. Transport phenomena in a steam-methanol reforming microreactor with internal heating. Int J Hydrogen Energy 2009;34(1): 314e22. [13] Karim A, Bravo J, Gorm D, Conant T, Datye A. Comparison of wall-coated and packed-bed reactors for steam reforming of methanol. Catal Today 2005;110(1):86e91.
[14] Karim A, Bravo J, Datye A. Nonisothermality in packed bed reactors for steam reforming of methanol. Appl Catal A 2005; 282(1):101e9. [15] Varesano A, Guaglio I, Saracco G, Maffettone PL. Dynamics of a methanol reformer for automotive applications. Ind Eng Chem Res 2005;44(4):759e68. [16] Hsueh CY, Chu HS, Yan WM, Chen CH. Numerical study of heat and mass transfer in a plate methanol steam micro reformer with methanol catalytic combustor. Int J Hydrogen Energy 2010;35(12):6227e38. [17] Arzamendi G, Dieguez PM, Montes M, Centeno MA, Odriozola JA, Gandia LM. Integration of methanol steam reforming and combustion in a microchannel reactor for H2 production: a CFD simulation study. Catal Today 2009;143(1e2):25e31. [18] Arzamendi G, Dieguez PM, Montes M, Odriozola JA, SousaAguiar EF, Gandia LM. Methane steam reforming in a microchannel reactor for GTL intensification: a computational fluid dynamics simulation study. Chem Eng Sci 2009;154(1e3):168e73. [19] Pan L, Wang S. Modeling of a compact plate-fin reformer for methanol steam reforming in fuel cell systems. Chem Eng J 2005;108(1):51e8. [20] Yan WM, Yang CH, Soong CY, Chen F, Mei SC. Experimental studies on optimal operating conditions for different flow field designs of PEM fuel cells. J Power Sources 2006;160(1):284e92. [21] Yan WM, Li HY, Chiu PC, Wang XD. Effects of serpentine flow field with outlet channel contraction on cell performance of proton exchange membrane fuel cells. J Power Sources 2008; 178(1):174e80. [22] Jang JH, Yan WM, Li HY, Tsai WC. Three-dimensional numerical study on cell performance and transport phenomena of PEM fuel cells with conventional flow fields. Int J Hydrogen Energy 2008;33(1):156e64. [23] Yan WM, Li HY, Tsai WC. Three-dimensional analysis of PEM fuel cells with different flow channel designs. J Electrochem Soc 2006;153(1):1984e91. [24] Jang JY, Huang YX, Cheng CH. The effects of geometric and operating conditions on the hydrogen production performance of a micro-methanol steam reformer. Chem Eng Sci 2010;65(20):5495e506. [25] Hsueh CY, Chu HS, Yan WM, Chen CH. Transport phenomena and performance of a plate methanol steam micro-reformer with serpentine flow field design. Appl Energy 2010;87(10):3137e47. [26] Kundu A, Jang JH, Lee HR, Kim SH, Gil JH, Jung CR, et al. MEMS-based micro-fuel processor for application in a cell phone. J Power Sources 2006;162(1):572e8. [27] White FM. Viscous fluid flow. 2nd ed. McGraw-Hill; 1991. [28] Ergun S. Fluid flow through packed columns. Chem Eng Prog 1952;48(2):89e94. [29] Peppley A, Amphlett JC, Kearns LM, Mann RF. Methanol steam reforming on CuZnOAl2O3 catalysts. Part 2. A comprehensive kinetic model. Appl Catal A 1999;179(1e2):31e49. [30] Mastalir A, Frank B, Szizybalski A, Soerijanto H, Deshpande A, Niederberger M, et al. Steam reforming of methanol over Cu/ ZrO2/CeO2 catalysts a kinetic study. J Catal 2005;230(2):464e75. [31] Pasel J, Emonts B, Peters R, Stolten D. A structured test reactor for the evaporation of methanol on the basis of a catalytic combustion. Catal Today 2001;69(1e4):193e200. [32] Won JY, Jun HK, Jeon MK, Woo SI. Performance of microchannel reactor combined with combustor for methanol steam reforming. Catal Today 2006;111(3):158e63. [33] Pattekar AV, Kothare Mv. A microreactor for hydrogen production in micro fuel cell applications. J Microelectromech Syst 2004;13(1):7e18.
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Three-dimensional analysis of a plate methanol steam microreformer and a methanol catalytic combustor with different flow channel designs Ching-Yi Hsueh a, Hsin-Sen Chu b, Wei-Mon Yan c,*, Guang-Ching Leu a, Jong-Ian Tsai a a
Network Operations Laboratory, Chunghwa Telecommunication Laborateries, Yang-Mei, Tao-Yuan 326, Taiwan, ROC Industrial Technology Research Institute, Chu-Tung, Hsin-Chu 310, Taiwan, ROC c Department of Greenergy, National University of Tainan, Tainan 700, Taiwan, ROC b
article info
abstract
Article history:
Three-dimensional models of a plate methanol steam micro-reformer and a methanol
Received 30 March 2011
catalytic combustor with parallel flow fields and serpentine flow fields have been estab-
Received in revised form
lished. The effects of the flow field design and the fuel flow rate on the methanol
7 July 2011
conversion and transport phenomena in the micro-reformer were investigated. The results
Accepted 23 July 2011
revealed that the methanol conversion of the micro-reformer with the serpentine flow field and the combustor with the serpentine flow field has been optimized as a result of improved thermal management in the micro-reformer with combustor. With a change in
Keywords:
flow field design from the micro-reformer and the combustor with parallel flow fields to the
Micro-reformer
micro-reformer and combustor with the serpentine flow fields a wall temperature increase
Catalytic combustor
from 225 C to 237 C was observed. The methanol conversion of the micro-reformer with
Methanol
the serpentine flow field and the combustor with the serpentine flow field could be
Flow field designs
improved by 23% relative to the employment of a parallel flow field. A numerical model provided an efficient way to characterize the transport phenomena within the microreformer and combustor; the results will benefit the future design of plate methanol steam micro-reformers with combustors. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
A methanol steam micro-reformer is a chemical device that drives the conversion of methanol to hydrogen. In a proton exchange membrane fuel cell (PEMFC), electricity and water are produced from the combination of hydrogen and oxygen in an electrochemical reaction. A small PEMFC with methanol steam micro-reformer can hold a high energy density, while operating with a low noise and pollution output; this makes it a potential candidate for portable electronic products in the near future. Therefore, the vast literature devoted to the
experiments for the methanol steam micro-reformer has been reviewed on several occasions [1e5]. A number of studies have been done on the transport phenomena and performance of the micro-reformer. In order to simplify the analysis, many studies only considered the plate micro-reformer, namely the catalytic combustor is not included in the analysis [6e10]. Hsueh and collaborators [6e8] developed a mathematical model for the plate methanol steam micro-reformer to investigate the effects of geometric and thermo-fluid parameters on performance as well as heat and mass transfer phenomena in micro-reformer channels.
* Corresponding author. Tel.: þ886 6 260 2251; fax: þ886 6 260 2205. E-mail address: [email protected] (W.-M. Yan). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.07.099
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RMD
Nomenclature Ci cp D Deff Dk Dp Ea H HB , HT HC HR HW DH I, J, K keff kf kp ks k1 k2 k3 k4 L Mi Mw,i p R RSR RrWSG
3
concentration of species i, mol m specific heat at constant pressure, J kg1 K1 hydraulic diameter, m effective mass diffusivity, m2 s1 mass diffusion coefficient, m2 s1 catalyst particle diameter (m) activation energy, J mol1 micro-reformer height, m steel height, m combustion flow channel, m reforming flow channel, m solid wall thickness, m Enthalpy of reaction, J mol1 grid points in the x, y and z directions, respectively effective thermal conductivity, W m1 K1 fluid phase thermal conductivity, W m1 K1 permeability, m2 solid medium thermal conductivity, W m1 K1 pre-exponential factor for steam reforming pre-exponential factor for the reverse water gas shift pre-exponential factor for decomposition reaction pre-exponential factor for combustion reaction flow channel length, m mole fraction of species i molecular weight of species i, kg mol1 pressure, Pa universal gas constant Arrhenius reaction rate coefficient for steam reforming, mol m3 s1 Arrhenius reaction rate coefficient for the reverse water gas shift, mol m3 s1
The tendencies of parameters (including longer channel length, smaller channel height, thicker catalyst layer, smaller aspect ratio, larger catalyst porosity, lower Reynolds number and higher wall temperature) to enhance the performance of the micro-reformer were identified. A model of the plate methane reformer to explore the temperature and gas distributions in the channels was developed by Yuan et al. [9,10]. The results indicated that small thermal conductivity of the catalyst layer and the solid walls have high reforming reaction rates. The effects of heat and mass transfer characteristics in a cylindrical mathematical model of a packed bed reformer are presented by several investigators [11e14]. Suh et al. [11,12] found that the internally heated reformer could improve methanol conversion. Karim et al. [13,14] showed the minimum reformer diameter had the highest catalyst activity and smallest temperature gradient. The theoretical modeling of steam reforming coupled with catalytic combustion for the plate reactors has been studied by several investigators [15e19]. The catalytic combustor supplied heat to the steam reforming reaction, and hydrogen was produced by the micro methanol steam reformer. A onedimensional transient mathematical model to study the transport phenomena in a methanol steam reformer with
Arrhenius reaction rate coefficient for decomposition reaction, mol m3 s1 RCombustion Arrhenius reaction rate coefficient for combustion reaction, mol m3 s1 T temperature, C inlet temperature, C T0 wall temperature, C Tw u, v, w velocity components in the x, y and z directions, respectively, m s1 Y Y ¼ y/HC þ dC þ HW þ dR þ HR x, y, z coordinates, m W micro-reformer width, m steel width, m W1 channel width, m W2 rib width, m W3 Greek symbols combustion catalyst layer thickness, m dC Reforming catalyst layer thickness, m dR ε porosity h methanol conversion s tortuousity of the porous medium m viscosity, kg m1 s1 r density, kg m3 rs catalyst density, kg m3 Subscripts eff effective u x-direction v y-direction w z-direction 0 inlet
a combustor was presented by Varesano et al. [15]. The transient characteristics of the reformer were examined in detail. Hsueh et al. [16] developed a three-dimensional numerical model of a micro-reformer with combustor to examine the effects of various flow configurations and the flow channel on the performance of the micro-reformer. Comparing the coand counter-current flows via numerical simulation, their results show that methanol conversion for counter-current flow could be improved by 10%. The results also showed that an improved micro-reformer performance was given by a lower reforming channel height. Arzamendi et al. [17,18] established a micro-channel model using thermal integration of a steam reformer and a catalytic combustor. Using the hydrogen produced by the reforming reaction from methanol and methane, the results showed the short diffusion distance and higher area to volume ratio required for using the micro reactor. The results also indicated that complete combustion takes place over a very short distance. The reforming fuel is rapidly heated, subsequently giving the reactor a more uniform temperature distribution. Pan and Wang [19] developed a plate-fin reformer which integrated endothermic and exothermic reactions into one unit. The combustor supplied the heat for the methanol steam reformer. Numerical
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 3 5 7 5 e1 3 5 8 6
simulation results accurately predicted the methanol conversion rate and gas distributions. They also indicated that the temperature distributions were much more uniform in the plate-fin reformer. In order to simplify the analysis, Varesano et al. [15] has considered the numerical model of methanol steam reformers, only including energy equation and concentration equations with chemical reaction. Furthermore, the continuity equation, momentum equation, energy equation and species equations with chemical reaction were employed in the reformer with the combustor by several researchers [16e19]. However, they have studied plate steam reformers with a parallel flow field which is attractive due to its simplicity. In this study, an attempt is made to examine the detailed fluid flow, heat and mass transfer coupled with chemical reactions in a three-dimensional computational model of the plate methanol micro-reformer with a methanol catalytic combustor for the serpentine and the parallel flow field.
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The flow field design in a fuel cell is one of the most important issues for a PEMFC. An appropriate flow field design in the fuel cell can improve the reactant transport, as well as thermal and water management. To this end, different flow field configurations, including parallel, serpentine and interdigitated have been developed. Many efforts have been devoted to optimize the flow field design to improve cell performance [20e23]. In recent years, several studies based on flow field design theory were applied to the plate methanol microreformer. Jang et al. [24] developed five inlet and outlet manifold configurations in a three-dimensional numerical model. The results showed how the configuration of a central inlet with two outlets could improve the methanol conversion to 23%. A plate methanol steam micro-reformer model with a serpentine flow field was built and its transport phenomena and performance was investigated and presented by Hsueh et al. [25]. Top plate heating with the serpentine flow field had a higher methanol conversion than with bottom plate heating.
a
L
HT HC C
y
H HW
x
Y=0.333
R
Y=0.167
HR 0
HB W1
b
z W2
W3
W
c
Fig. 1 e (a) Schematic diagram of a plate methanol steam micro-reformer with methanol catalytic combustor, (b) parallel flow field and (c) serpentine flow field.
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Different types of flow field designs for plate methanol microreformers have been used to achieve more efficient methanol conversion. Kundu et al. [26] used different flow configurations, including serpentine and parallel flow fields, to improve plate methanol reformer performance. The results showed the serpentine flow field had a better durability than the parallel flow field. From the literature cited above, it was shown that methanol conversion can be enhanced by a suitable flow channel design. However, only limited work has been done to investigate the effect of different flow field designs on the performance of the plate methanol micro-reformer with methanol catalytic combustor. Therefore, the objective of this paper is to establish a three-dimensional computational model of the plate methanol micro-reformer with a methanol catalytic combustor to investigate the performance and transport phenomena of the micro-reformer with various flow fields (parallel flow field and serpentine flow field).
2.
Analysis
2 vv vv vv vp v v v2 v v2 v εr u þ v þ w þ 2 þ 2 þ Sv ¼ ε þ εm 2 vx vy vz vy vx vy vz
(3)
Z-momentum equation: 2 vw vw vw vp v w v2 w v2 w εr u þ v þ w þ Sw þ þ ¼ ε þ εm vx vy vz vz vx2 vy2 vz2
(4)
In the above equations, ε is the porosity of the medium, and the mixture viscosity mmix is [27] 5 X
mmix ¼
i¼1
Mi mi P5 j¼1 Mj fij
(5)
where 2
fij ¼
In this study, three-dimensional computational models with various flow fields have been established for a methanol steam micro-reformer with a methanol catalytic combustor. The flow fields in the methanol steam micro-reformer and methanol catalytic combustor include the parallel flow field and the serpentine flow field. The parallel flow field has five parallel channels and the serpentine flow field has one channel with four turns. A schematic illustration of these flow fields and associated coordinate system are shown in Fig. 1. The parallel flow field has five flow channels and each channel is 40 mm in length. The serpentine flow field has one flow channel, the total flow channel length is five times the length of a channel in the parallel flow field, and there are four turning points. In this study, a constant flow rate approach is utilized to investigate the effect of the flow field on the performance of the microreformer. The letters u, v, and w are the velocity components in the x-, y-, and z-directions, respectively. The reactor consists of a solid wall, a steam reforming flow channel, a steam reforming catalyst layer, a catalytic combustion catalyst layer and a catalytic combustion flow channel. To simplify the analysis, it is assumed that, the flow is in a steady state, the inlet fuel is an ideal gas, the flow is laminar and incompressible, the catalyst layer is isotropic, the chemical reaction occurs only in the catalyst layer and thermal radiation and conduction in the gas phase are negligible compared to convection. The transport equations for the three-dimensional model are given as:
X
6 41 þ
32 !1 1 mi 2 Mw;j 4 7 5 mj Mw;i
1 Mw;i 2 8 1þ Mw;j
i
(6)
In the momentum equations, Su, Sv and Sw are correction terms of the reactant gas flow in a porous material in the catalyst layer. In this work, the flow channel is not a porous medium, therefore, Su, Sv and Sw are zero in the flow channel. However, the catalyst layer is a porous medium, the source terms, Su, Sv and Sw account for the Ergun equations [28] in the x-, y- and z-directions, respectively. The parameter kp is the permeability and b is the inertial loss coefficient in each component direction. ffi mu burpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u2 þ v2 þ w2 Su ¼ kp 2
(7)
ffi mv bvrpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Sv ¼ u2 þ v2 þ w2 kp 2
(8)
ffi mw bwrpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Sw ¼ u2 þ v2 þ w2 kp 2
(9)
where kp ¼
b¼
D2p ε3 150ð1 εÞ2
3:5ð1 εÞ Dp ε3
(10)
(11)
and where Dp is the catalyst particle diameter. Species equation:
Continuity equation: vu vv vw þ þ ¼0 r vx vy vz
Y-momentum equation:
(1)
2 vmi vmi vmi v mi v2 mi v2 mi u þv þw ¼Deff þ 2 þ 2 2 vx vy vz vx vy vz
(12)
þð1εÞrs Sc X-momentum equation:
2 vu vu vu vp v u v2 u v2 u εr u þ v þ w þ 2 þ 2 þ Su ¼ ε þ εm 2 vx vy vz vx vx vy vz
(2)
In the species equation, mi denotes the mass fraction of the ith species, where the various species are CH3OH, H2O, H2, CO2, CO and O2. In these expressions, the concentrations of CH3OH, H2O, H2, CO2, CO are calculated on the steam reforming side and CH3OH, H2O, CO2, O2 are calculated on the
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 3 5 7 5 e1 3 5 8 6
combustion side. Deff is the effective diffusion coefficient based on the StefaneMaxwell equations [27]. Eq. (13) is employed to describe the influence of the porosity on the diffusion coefficient. Deff ¼ Dk εs
(13)
The diffusion coefficient Dk for the methanol steam microreformer was derived from the StefaneMaxwell equations which were used to calculate the mean effective binary diffusivity [17]. Sc represents the source term due to the chemical reaction in the catalyst layer. In this study, there was no chemical reaction in the flow channel. Therefore, Sc was zero in the flow channel. In the catalyst layer, Sc differs according to the chemical reactions in the catalyst layer. The source term of the species equation, Sc, can be described by the following equations: Mw;i ðRSR þ RrWGS þ RMD Þ l00i l0j for steam reforming Sc ¼ Mw;i ðRCombustion Þ l00i l0j for combustion reaction
(14)
where Mw,i is the molecular weight of species i, RSR, RrWGS, RMD and RCombustion are the Arrhenius molar rates in the reaction, l}i and lj are the stoichiometric coefficients for reaction i and product j, respectively, in the reaction. According to the chemical kinetics of Pepply et al. [29], the methanol steam reforming reaction consists of three overall reactions: one is the primary process in the methanol steam reforming and the others are the decomposition reaction and the wateregas shift reaction. Therefore, the steam reforming reaction, Eq. (15), the reverse wateregas shift reaction, Eq. (16), and the decomposition reaction, Eq. (17), are each considered in the present study. k1
CH3 OH þ H2 O ! CO2 þ 3H2 k1
k2
CO2 þ H2 ! CO þ H2 O k2
k3
CH3 OH/CO þ 2H2
(15)
(16)
(17)
In this study, to simplify the analysis, RSR, RrWGS and RMD are the methanol steam reforming reaction bases on the Mastalir et al. [30]. The Arrhenius equation is employed to calculate chemical reaction rate coefficients: Ea Ea 0:4 k C exp C C exp RSR ¼ k1 C0:6 1 CO H CH3 OH H2 O 2 2 RT RT
(18)
Ea Ea RrWGS ¼ k2 CCO2 CH2 exp k2 CCO2 CH2 O exp RT RT
(19)
Ea RMD ¼ k3 C1:3 exp CH3 OH RT
(20)
where the steam reforming reaction and reverse wateregas shift reaction are reversible reactions and the decomposition reaction is a non-reversible reaction. The constants k1, k2 and k3 are the forward rate constants, and the constant k1 and k2 are the backward rate constants.
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The reaction of the catalytic combustion catalyst layer can be represented by Eq. (21). The reaction rate coefficient of the methanol combustion reaction over the Pt/Al2O3 catalyst was calculated with Eq. (22), as proposed by Pasel et al. [31] k4
CH3 OH þ 1:5O2 /CO2 þ 2H2 O
(21)
Ea exp RCombustion ¼ k4 C1:3 CH3 OH RT
(22)
To evaluate the local temperature distributions, the energy equations must be solved. Energy equation: 2 vT vT vT v T v2 T v2 T þ ð1 εÞrs St þ þ ¼ keff rcp u þ v þ w vx vy vz vx2 vy2 vz2
(23)
keff is the modified effective thermal conductivity of the porous medium which is given by keff ¼ εkf þ ð1 εÞks
(24)
where kf is the fluid phase thermal conductivity, ks is the solid medium thermal conductivity and ε is the porosity of the medium. St in the channel is zero. The catalyst layer experiences exothermic and endothermic chemical reactions, so the source term St in the energy equation due to the chemical reactions is determined by
St ¼
ðDHSR RSR þDHrWGS RrWGS þDHMD RMD Þ forsteamreforming ðDHCombustion RCombustion Þ forcombustionreaction (25)
The energy equation of the solid wall can be calculated by v2 T v2 T v2 T þ þ ¼0 vx2 vy2 vz2
(26)
The boundary conditions at the inlet, outlet, wall, and the interface between the flow channel and the catalyst layer were as follows: the inlet flow velocity is constant, the inlet gas composition is constant and the inlet temperature is constant. At the flow channel and the catalyst layer, there is fully developed flow. At the interface between the solid wall and the insulated walls, the temperature gradients were zero. At the interface between the flow channel and the solid wall, no slip and zero fluxes held the velocities and the concentration gradients at zero. For the interface between the flow channel and the catalyst layer, the velocities, temperatures, species concentrations and species fluxes are continuous. The governing equations were made discrete by employing a finite volume scheme and subsequently solved using the commercial fluid dynamics program Fluent. Details of the numerical solutions have been given in previous studies [16]. The convergence criteria for the normalized residuals for each variable were restricted to less than 106. The dimensions of the plate methanol steam micro-reformer with a methanol catalytic combustor were 40 mm (L) 15 mm (W ) 2.5 mm (H ), the inlet and outlet cross-sections of the flow channel were 1.0 mm 0.5 mm, and the rib width (W3) was 2 mm. The thickness of the catalyst layer was set to 0.05 mm. The operating pressure and temperature of both micro-reformer and combustor were 1 atm and 393 C, respectively, the inlet flow
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320
Table 1 e Parameters used in this study.
300 280
o
260
w
4 102 5.0 105 5.0 105 4.5 104 4.5 104 1 103 1 103 2 103 120 1 1480 0.3
T ( C)
Length L (m) Combustion catalyst layer thickness dC (m) Reforming catalyst layer thickness dR (m) Combustion flow channel HC (m) Reforming flow channel HR (m) Steel width W1 (m) Channel width W2 (m) Rib width W3 (m) Average inlet temperature ( C) Operating pressure (atm) Catalyst density (kg m3) [13] Catalyst thermal conductivity (W m1 k1) [13] Catalyst layer porosity [33] Catalyst permeability (m2) [33] Mass diffusion coefficient (m2 s1) [17] Activation energy for steam reforming (J mol1) [30] Activation energy for the reverse water gas shift (J mol1) [30] Activation energy for decomposition reaction (J mol1) [30] Activation energy for combustion reaction (J mol1) [31] Pre-exponential factor for steam reforming Pre-exponential factor for the reverse water gas shift Pre-exponential factor for decomposition reaction Pre-exponential factor for combustion reaction
240 220
180
0.3
0.35
0.4
u
0.38 2.379 1012 6.8 105
0.45
-1
0,C
(m s )
100
1.09 105
80
1.15 105
60
1.42 10
Present Results Experimental Study of Won et al. [32]
200
5
40
1.3 104 20
Present Results Experimental Study of Won et al. [32]
9.55 1012 0 200
1.65 1013
220
240
260
280
300
o
T ( C) w
1.65 1013 5.8 105
rates at the reforming side and at the combustion side were 7.5 cm3 min1 and 75 cm3 min1 respectively. The parallel flow field and the serpentine flow field have the same inlet flow rates. The geometrical dimensions and parameters used are listed in Table 1. In order to simplify the analysis, three grid systems were evaluated for the single channel of the plate methanol steam micro-reformer with combustor. The grid numbers in the x-, yand z-directions were 151 71 10, 161 76 13 and 171 81 16, respectively. To examine the independence of the grid predictions, three grid systems were considered and their influences on the prediction of local temperature distributions for a typical case are presented in Table 2. It was found that the maximum deviation among the computations using grids of 151 71 10, 161 76 13 and 171 81 16 was less than 0.76% and the results on the 161 76 13 and the 171 81 16 grids were relatively close. Therefore, the parallel flow field grid 161 76 61 was chosen for the simulation model in making a tradeoff between accuracy and CPU time.
Fig. 2 e Comparison of theoretical simulation of present results with previous experimental data by Won et al. [32].
Fig. 2 gives a comparison of calculated methanol conversion and wall temperature with experimental data [32]. Only a small discrepancy was found between the numerical results and the experimental data. Preliminary runs confirmed that the micro-reformer performance and gas transport phenomena could be accurately predicted using our simulation. This model has the potential to reduce the design time of a plate methanol steam micro-reformer with combustor.
3.
Results and discussion
This paper aimed to establish three-dimensional computational models of a plate methanol steam micro-reformer with a methanol catalytic combustor to investigate the performance and transport phenomena of parallel and serpentine flow fields. Due to discrepancies in the flow channel design, flow channel length and the turning points among various flow field designs; the fuel consumption and temperature
Table 2 e Temperature distributions ( C) for the various grid tests at different axial locations. X IJK 151 71 10 161 76 13 171 81 16
0.125 224.9 224.0 223.2
0.250 225.6 224.7 223.9
0.375 225.9 225.1 224.3
0.500 226.1 225.3 224.5
0.625 226.2 225.4 224.6
0.750 226.2 225.4 224.6
0.875 226.3 225.4 224.6
1.000 226.3 225.4 224.6
Fig. 3 e The velocity distributions on the middle cross-section of the reforming channel (Y [ 0.167) for reformer and combustor with various flow field designs.
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distributions inside the micro-reformer with combustor were found to be different. The effects of the flow field designs on the velocity, temperature, CH3OH mole fraction, H2 mole fraction and CO mole fraction distributions were investigated and are presented in this study. The velocity distribution on the middle cross-section of the reforming channel (Y ¼ 0.167) is presented in Fig.3. In the serpentine flow field, the flow channel values were found to be one-fifth of that of the parallel flow field. The fuel velocity increases due to a greater channel convection effect. It can be seen that recirculation flow takes place at the turning points of the flow channels due to the change of velocity direction, generating shear stresses, which increase fuel consumption. The temperature distributions on the top cross-section of the reforming channel (Y ¼ 0.333) are presented in Fig. 4. A constant flow rate was employed in this analysis. The temperature distribution along the channel was seen to increase as a consequence of the chemical reaction along the flow channels. For the combustor, the results indicated a higher temperature for the combustor with serpentine flow field. This can be made plausible by noting the fact that the serpentine flow field had a higher fuel flow velocity than the parallel flow field, and that the fuel experiences several turning points before reaching the outlet; this increases the energy released. In regards to the reformer, a higher temperature distribution was noted in the serpentine flow field. This may be because the smaller outlet area decreased the heat leaving the flow channel. Therefore, the serpentine flow field provided better thermal management than a parallel flow field. The combustor with serpentine flow field and the microreformer with serpentine flow field have the highest temperature distribution for each of the channel designs. Fig. 5 presents the CH3OH mole fraction distributions on the middle cross-section of the reforming channel (Y ¼ 0.167) for the various flow fields. It shows that the CH3OH mole fraction decreased along the channel for the four channel designs due to the methanol steam reforming reaction. By comparing the CH3OH mole fraction distributions, the highest methanol concentration was noted for the combustor with a parallel flow field and the micro-reformer with a parallel flow field. This implies that the chemical reaction rate was slowest for the combustor with a parallel flow field and the micro-reformer with a parallel flow field due their having the lowest temperature distribution. The methanol conversion rates at the channel exits were; 52%, 62%, 68%, and 79% for Fig. 5(a) the combustor with parallel flow field and the microreformer with parallel flow field, Fig. 5(c) the serpentine flow field with combustor and the micro-reformer with parallel flow field, Fig. 5(b) the combustor with parallel flow field and the micro-reformer with serpentine flow field, Fig. 5(d) the combustor with serpentine flow field and the micro-reformer with serpentine flow field, respectively. Therefore, it was expected that the methanol conversion would be highest for the combustor with a serpentine flow field and the microreformer with a serpentine flow field. The distributions of the H2 mole fraction on the middle cross-section of the reforming channel (Y ¼ 0.167) are shown in Fig. 6 for the various flow fields. A higher H2 mole fraction along the flow channel represents greater methanol conversion. Thus, the variation of the H2 fraction was opposite to
Fig. 4 e The temperature distributions on the top crosssection of the reforming channel (Y [ 0.333) for reformer and combustor with various flow field designs.
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Fig. 5 e The CH3OH mole fraction distributions on the middle cross-section of the reforming channel (Y [ 0.167) for reformer and combustor with various flow field designs.
Fig. 6 e The H2 mole fraction distributions on the middle cross-section of the reforming channel (Y [ 0.167) for reformer and combustor with various flow field designs.
that of the CH3OH mole fraction. For the combustor, the serpentine flow field had a greater temperature distribution than the parallel flow field. In regards to the reformer, the serpentine flow field had a greater chemical reactivity than
the parallel flow field due since the fuel remained in the channel longer and thus the serpentine flow field provided a better methanol conversion. Therefore, the highest H2 mole fraction was found for the combustor with a serpentine flow
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confirmed the common belief that a micro-reformer with H2 production indicates a higher CO concentration. For the combustor with serpentine flow field and the micro-reformer with a serpentine flow field, the CO mole fraction was 746 ppm at the outlet of the reforming channel. The inlet flow rate (Q0,R) on the micro-reformer side is a significant issue to be considered when choosing the flow field designs. Fig. 8(a) shows the effects of the inlet flow rate (Q0,R) on the micro-reformer side on the wall temperature of the reforming channel for various flow field designs. The results revealed that the wall temperature was increased by the decreased inlet flow rate (Q0,R) on the micro-reformer. This was due to a higher inlet flow rate (Q0,R) significantly increasing the heat leaving the flow channel, which reduced the rise in temperature. The combustor with the serpentine flow field and the micro-reformer with the serpentine flow field had the highest wall temperature of the reforming channel of all other flow fields. In addition, it is seen in Fig. 8(b) that a higher methanol conversion (h) increases with a lower inlet flow rate (Q0,R) due to a longer gas residence time and a higher temperature. The methanol conversion of the serpentine flow field with combustor and the serpentine flow field with micro-reformer was the best due to a higher wall temperature. The effects of the flow field designs on the inlet flow rate (Q0,C) of the combustor and wall temperature of the reforming channel are presented in Fig. 9. It indicates that the
a
300 Combustor (Parallel Flow Field), Reformer (Parallel Flow Field) Combustor (Parallel Flow Field), Reformer (Serpentine Flow Field) Combustor (Serpentine Flow Field), Reformer (Parallel Flow Field) Combustor (Serpentine Flow Field), Reformer (Serpentine Flow Field)
260
w
o
T ( C)
280
240
220
200 2
4
6
Q
b
8 3
0,R
10
12
-1
(cm min )
100 (b) 80
(%)
60
40
Fig. 7 e The CO mole fraction distributions on the middle cross-section of the reforming channel (Y [ 0.167) for reformer and combustor with various flow field designs.
Combustor (Parallel Flow Field), Reformer (Parallel Flow Field) Combustor (Parallel Flow Field), Reformer (Serpentine Flow Field) Combustor (Serpentine Flow Field), Reformer (Parallel Flow Field) Combustor (Serpentine Flow Field), Reformer (Serpentine Flow Field)
20
0 2
field and the micro-reformer with a serpentine flow field. Fig. 7 presents the effects of the various flow field designs on the CO mole fraction distributions on the middle cross-section of the reforming channel (Y ¼ 0.167). It was found that the CO distributions had the same trends as the H2 distributions. This
4
6
8
Q
(cm min )
3
0,R
10
12
-1
Fig. 8 e Effects of the inlet flow rate (Q0,R) of the reformer and various flow field designs on (a) wall temperature and (b) methanol conversion.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 3 5 7 5 e1 3 5 8 6
a
260
220
w
o
T ( C)
240
200 Combustor (Parallel Flow Field), Reformer (Parallel Flow Field) Combustor (Parallel Flow Field), Reformer (Serpentine Flow Field)
180
Combustor (Serpentine Flow Field), Reformer (Parallel Flow Field) Combustor (Serpentine Flow Field), Reformer (Serpentine Flow Field)
160 20
40
60
Q
b
80 3
0,C
100
120
-1
(cm min )
100
80
60
40
Combustor (Parallel Flow Field), Reformer (Parallel Flow Field) Combustor (Parallel Flow Field), Reformer (Serpentine Flow Field) Combustor (Serpentine Flow Field), Reformer (Parallel Flow Field) Combustor (Serpentine Flow Field), Reformer (Serpentine Flow Field)
20
0 20
40
60
Q
80 3
0,C
100
120
-1
(cm min )
Fig. 9 e Effects of inlet flow rate (Q0,C) of the combustor and various flow field designs on (a) wall temperature and (b) methanol conversion.
temperature increased with an increase in the inlet flow rate (Q0,C) on the combustor side due to a higher inlet flow rate. This is because greater combustion energy was released due to a higher inlet flow rate. The wall temperature was observed to increase in the following order: the combustor with parallel flow field and the micro-reformer with parallel flow field; the combustor with serpentine flow field and the micro-reformer with parallel flow field; the combustor with parallel flow field
10
6
H2
3
-1
Q (cm min )
8
4 Combustor (Parallel Flow Field), Reformer (Parallel Flow Field) Combustor (Parallel Flow Field), Reformer (Serpentine Flow Field) Combustor (Serpentine Flow Field), Reformer (Parallel Flow Field) Combustor (Serpentine Flow Field), Reformer (Serpentine Flow Field)
2
0 2
4
6
Q
3
0,R
8
-1
10
12
(cm min )
Fig. 10 e Effects of the inlet flow rate (Q0,R) of the reformer and various flow field designs on the H2 production rate.
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and the micro-reformer with serpentine flow field; the combustor with serpentine flow field and the micro-reformer with serpentine flow field. This was due to improvements in heat transport within the flow field design. In addition, the effects of the flow field designs on the inlet flow rate (Q0,C) of the combustor and methanol conversion in the plate methanol steam micro-reformer are shown in Fig. 9(b). In regards to methanol conversion, the results indicate that the methanol conversion increased as the inlet flow rate (Q0,C) of the combustion channel increased. The highest efficiency methanol conversion was enhanced by the combustor with serpentine flow field and the micro-reformer with serpentine flow field. Therefore, it is concluded that increasing the number of corners and channel length to the various flow fields can effectively raise the temperature distribution and thereby enhance the methanol conversion. Fig. 10 demonstrates the effects of the inlet flow rate (Q0,R) on the micro-reformer side on the H2 production rate of the channel for various flow fields. Fig. 10 implies that the H2 production rate (QH2) increased with an increasing inlet flow rate (Q0,R) on the micro-reformer side due to a higher inlet flow rate. By comparing Figs. 8 and 10, QH2 increased and h decreased with an increase in Q0,R. Therefore, it is important to note that a higher inlet flow rate (Q0,R) on the microreformer side will not necessarily provide a better H2 production rate. When the methanol conversion was too small, a higher inlet flow rate (Q0,R) provided a lower H2 production rate. The combustor with serpentine flow field and the micro-reformer with serpentine flow field was the best design when considering the H2 production rate. When the combustor with serpentine flow field and the micro-reformer with serpentine flow field for a Q0,R of 9 cm3 min1, the methanol conversion was 67%, and the H2 production rate was 8 cm3 min1.
4.
Conclusions
Three-dimensional numerical models of a methanol steam micro-reformer with a methanol catalytic combustion chamber have been developed in this analysis. The flow fields including the parallel flow field and the serpentine flow fields were investigated. The temperature, methanol mole fraction, hydrogen mole fraction and CO mole fraction for the various flow fields have been studied in detail. 1. A higher temperature distribution was found for the plate methanol steam micro-reformer with the serpentine flow field as well as the methanol catalytic combustor with the serpentine flow field. 2. The plate methanol steam micro-reformer with the serpentine flow field and the methanol catalytic combustor with the serpentine flow field were found to have a better methanol conversion than any other flow field design in this study. 3. With a change in flow field design from the micro-reformer and the combustor with parallel flow fields to the microreformer and combustor with the serpentine flow fields a wall temperature increase from 225 C to 237 C was observed.
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4 .The methanol conversion for the micro-reformer with the serpentine flow field and the combustor with the serpentine flow field could be improved by 23% relative to that of the micro-reformer with the parallel flow field and the combustor with the parallel flow field.
Acknowledgment The study was supported by the National Science Council, the Republic of China, through the grants NSC 97-2221-E-211-015MY2.
references
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