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Methanol reformation for hydrogen productionfrom a single channel with cavities

Prashant Nehe, Sudarshan Kumar*

Department of Aerospace Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India

a r t i c l e i n f o

Article history:

Received 12 December 2012

Received in revised form

27 July 2013

Accepted 30 July 2013

Available online 24 August 2013

Keywords:

Cavity type reformer

Hydrogen production

Methanol conversion rate

Plate type microreformer

* Corresponding author. Tel.: þ91 22 2576 71E-mail addresses: prashantnehe@yahoo.c

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.07.1

a b s t r a c t

This paper proposes a novel design concept to enhance the methanol conversion rate in a

single channel plate type microreformer with cavities. Detailed numerical studies have

been carried out to understand the steam reforming of methanol for hydrogen production.

The effects of operating parameters such as steam-to-methanol molar ratio, reforming

temperature, reformer gas hourly space velocity (GHSV), channel wall conductivity, wall

thickness and catalyst layer thickness on reforming characteristics are investigated. The

effect of cavities on microreformer performance is discussed in terms of cavity aspect ratio

and its spacing. For a reforming temperature of 250 �C, steamemethanol molar ratio of 1.1,

average inlet fluid temperature of 120 �C and catalyst thickness of 30 mm, a methanol

conversion of w98% with product gases consisting of 75% H2, 23% CO2 and 928 ppm CO

have been obtained at the outlet of the channel. Present studies show that higher methanol

conversion rates can be achieved within a shorter channel length with cavities. The pro-

posed design can overcome the issue of shape and size of manifolds and flow equi-

distribution for multiple microchannels type design and also suitable from fabrication

viewpoint and practical applications.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction membrane (PEM) fuel cells have the potential of providing

The enhancement in the performance of many portable

electronic devices in terms of miniaturization and increased

battery life has become important during the last decade.

Therefore, portable power sources capable of delivering power

in 0.1e100 W range are being actively researched for pro-

spective uses in combat situations as well as for electronic

devices, such as laptops and cell phones. Recent progress to-

wards the development of fuel cells proposes an alternative

power source due to their high-energy efficiency and eco-

friendly nature [1]. Fuel cells need a continuous supply of

hydrogen gas for their operation and therefore hydrogen is

called as a fuel for future. Hydrogen based proton exchange

24; fax: þ91 22 2572 2602.om (P. Nehe), sudar@aer2013, Hydrogen Energy P19

energy storage densities several times higher than those

possible using current state-of-the-art lithium-ion batteries

[2]. Although, PEMFCs have higher energy density, they need

to carry enough hydrogen fuel for sustained

operation. Therefore, researchers have been actively pursuing

the area of hydrogen production from various sources such as

reformation from methanol and fossil fuels. Methanol is a

highly suitable liquid fuel for onboard production of hydrogen,

offering a high hydrogenecarbon ratio (¼4), being liquid at

room temperature, biodegradable, free from sulfur, absence of

carbonecarbon bonds and its reactivity allowing reformation

at relatively lower temperatures (200e350 �C). This low

cracking temperature of methanol results in lower carbon

o.iitb.ac.in (S. Kumar).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

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 en e r g y 3 8 ( 2 0 1 3 ) 1 3 2 1 6e1 3 2 2 9 13217

monoxide (CO) emissions which is quite important for the

operation of PEMFCs because CO poisons the PEMFC anode

(which is required to be less than 10 ppm).

For the activation of a methanol steam reforming reaction

in the microreformers, energy must be supplied from an

external heat source because it is an endothermic reaction.

Therefore, heat transfer plays an important role in the

reforming process [3,4]. Microreformers have the advantage of

higher surface to volume ratio as compared to the traditional

steam reformers. Other advantages include flow uniformity

and long flow residence times [5e7]. There are two types of

reformers based on the catalyst layout, the conventional

packed-bed type and reformers using a wall coated or sus-

pended catalyst layer. Wall coated reformers have an advan-

tage of low pumping power without compromising on

reforming performance. Many experimental studies on plate

type methanol steam microreformers have been reported in

the literature. Several investigators [8e11] have used electric

heaters for providing heat for the endothermic steam

reforming reaction. Lim et al. [10] have experimentally

examined the methanol conversion and hydrogen production

from plate-type microchannel based microreformer. Their

results showed approximately 78% conversion of methanol

and 3 L h�1 hydrogen production rate. A high-performance Cu/

ZnO/Al2O3 catalyst developed by Kawamura et al. [12] allowed

hydrogen production at a comparatively lower temperature

than that of commercial catalysts.

Modeling and numerical simulations are frequently used

to obtain a better understanding of the effect of geometric

parameters and thermo-fluid processes on the performance of

methanol microreformers. Several numerical models report

one dimensional analysis describing themethanol conversion

and the heat and mass transport phenomena [13e17]. There

are many two-dimensional [18e20] and three-dimensional

[21e24] simulations of methanol reformers in the literature

which explicates the effects of various flow configurations on

the performance of the microreformers. Some researchers

have reported the optimization of the flow configuration and

flow field to improve the methanol conversion [25e27]. They

have used parallel flow field or serpentine flow field for the

investigation and found some improvement in the methanol

conversion.

When the reformer volume is reduced, issues such as

reformer geometry, flow pattern inside the reformer, resi-

dence time, fluid mixing and thermal management becomes

important. For higher methanol conversion, the control of

temperature and species concentration distribution by opti-

mizing the catalyst layout or reformer design and heat supply

in the reformer need to be addressed. The geometric design of

a reformer is one of the most vital issues. Proper reformer

geometry can advance the reactant gas transport and the ef-

ficiency of thermal management. In the plate type microre-

formers, multiple channels are typically implemented to

increase the residence time and contact area between the

catalyst and the reactant gases. Reforming reaction is not

uniform in these microreformers. Microchannels in the cen-

tral part always exhibit better reforming performance. Chen

et al. [28] found that such a plate type design of the methanol

microreformer may cause nonuniform reaction rates in each

microchannel and affect its reforming performance

significantly. Besides this, in case of microreformers with

multiple channels and multiple stacks placed one above the

other, two more difficulties faced are feed stream branching

with uniform flow rate for each stack and catalyst replace-

ment after catalyst deactivation. Therefore, a novel single

channel based configuration with multiple cavities is pro-

posed in this plate type microreformer design. Hence, the

objective of this paper is to investigate the performance of a

wall-coated plate type microreformer with cavities and the

effects of cavities on methanol conversion rate. The effect of

catalyst thickness and cavity aspect ratio on the flow and

temperature distribution for the improvement in methanol

conversion is investigated.

2. Numerical model and formulation

Fig. 1 shows a two dimensional geometry of the physical

model and a typical microreformer plate. Two microreformer

plates, mirror images of each other and with multiple cavities

are placed above each other to form a single channel of a plate

type microreformer. The mixture of methanol and steam en-

ters themicroreformer and then the steam reforming reaction

occurs at the catalyst layer. A homogenous layer of 30 mm

thickness of a commercially available Cu/ZnO/Al2O3 catalyst

is uniformly deposited on the fins of both the microreformer

plates. The catalyst layer thickness (dC), the center-to-center

spacing between adjacent cavities (s) and the cavity depth

(d ) are the variables for parametric studies. Relative cavity

depth is defined as the ratio of the cavity depth (d ) to channel

height (H ), d* ¼ d/H. The cavity geometric parameters are

defined relative to the channel height (H ) and the relative

cavity depth range is varied from 50 to 250% of H in this study.

Table 1 indicates the different cases of relative cavity depth

and cavity spacing considered for the study.

To ensure the validity of present numerical studies and

continuum model, the Knudsen number (Kn) has been evalu-

ated as

Kn ¼ l

l(1)

Knudsen numbers vary from 1.16 � 10�4 to 1.43 � 10�4.These values are relatively much lower than 0.001 [29] indi-

cating the validity of the continuum model and the Naviere-

Stokes equations with no-slip boundary conditions for the

systems considered in this study. Reynolds number has been

observed to vary from 2 to 18 and laminar flow dominates the

present system with viscous forces playing a dominant role.

2.1. Assumptions

Following assumptions have been made to simplify the nu-

merical studies for the analysis.

(1) The flow is steady and laminar.

(2) Ideal gas assumption and liquid methanol entering the

inlet is entirely vaporized to gas phase instantly.

(3) The gas flow in the microreformer is incompressible.

(4) The catalyst layer is a porous medium with homogenous

porosity and permeability.

Fig. 1 e (a) Schematic diagram of the physical model, (b) a typical microreformer plate.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 2 1 6e1 3 2 2 913218

(5) The chemical reaction takes place only in the catalyst

layer.

(6) Thermal radiation is neglected as compared to convection

and conduction.

(7) The catalyst layer is in local thermal equilibrium with the

adjacent gas mixture.

2.2. Governing equations

The conservation equations ofmass, momentum, energy, and

chemical species are solved. In the gas phase region, all these

equations are valid and in the solid region, energy conserva-

tion equation is solved.

V,�rV

� ¼ 0 (2)

V,V�rV

� ¼ �Vpþ mV2V þ Si (3)

Here the dynamic viscosity of the gas mixture is calculated

based on the ChapmaneEnskog theory for multi-component

gas mixtures at low density [30].

mmix ¼X5

i¼1

XimiP5j¼1 Xj4ij

(4)

Si is the source term, zero in the channel flow region and its

value in the porous catalyst layer and can be expressed as [31]

Si ¼ �mVkp� br

2

��V��V (5)

Table 1 e Summary of the various geometric parametersof the cavities considered for the present study.

Relative cavity depth d* 0.5, 1, 2, 2.5

Cavity spacing s/H 5.62, 2.81, 1.87

Cavity width w/H 0.67

The model of porous media has been used to simulate the

catalyst layer and in the above expression kp is the perme-

ability and b is the inertial loss coefficient.

V,�rVcpT

� ¼ V,�keffVT

�� ð1� εÞrðrRDHR þ rWGSDHWGS þ rDDHDÞ(6)

The term keff is the effective thermal conductivity of the

catalyst layer to take into account effect of porous medium

and is defined as,

keff ¼ εkf þ ð1� εÞks (7)

where kf and ks are the fluid and solid thermal conductivities

respectively. DHR and DHD are the enthalpies of reforming

reaction and decomposition reaction respectively.

V,�rVYi

�¼V,�DeffrVYi

�þð1� εÞr�MiðrRþ rWGSþ rDÞ�yi� yj

��(8)

where Yi is the mass fraction of species i like CH3OH, H2O, H2,

CO2, CO and Mi is the molecular weight of species i. In the

above equation yi and yj are the stoichiometric coefficients for

reaction i and product j respectively in the reaction and Deff is

the effective mass diffusion coefficient and calculated using

Eq. (9) in which the binary mass diffusion coefficient, Dij is

obtained using the method proposed by Reid et al. [32]. The

last term is the source termdue to the chemical reaction in the

catalyst layer and this term is zero in the gas phase as no re-

action occurs in the flow channel.

Deff; i ¼ 1� XiPNjsi Dij

(9)

For the solid wall of the microreformer, there is no mass

transport and reaction, the energy conservation equation is

V2T ¼ 0 (10)

2.3. Chemical reaction modeling

When Cu/ZnO/Al2O3 is used as a catalyst, the methanol steam

reforming reaction chemical kinetics consists of three overall

Table 2 e Geometric conditions, flow conditions andkinetic parameters used in this study.

Parameter Value

Flow channel length L (m) 4.5 � 10�2

Flow channel height H (m) 8.0 � 10�4

Flow channel wall

thickness Wt (m)

4.0 � 10�3

Catalyst layer

thickness (dC) (m)

3.0 � 10�5

Inlet average

temperature T0 (�C)120

Flow rate of entering

liquid mixture _Q(cm3 h�1)6-100

Steamemethanol

molar ratio g

0.8e1.3

Mass fraction of methanol 0.618

Mass fraction of water 0.382

Operating pressure (atm) 1

Activation energy for

steam reforming

(J mol�1) [35]

1.09 � 105

Activation energy for

reverse wateregas shift

(J mol�1) [35]

1.15 � 105

Activation energy for

decomposition reaction

(J mol�1) [35]

1.42 � 105

Catalyst density rc

(kg m�3) [20]1480

Catalyst layer

porosity ε [15]

0.38

Catalyst permeability

kp (m2) [15]

2.379 � 10�12

Mass diffusion coefficient

D (m2 s�1) [20]6.8 � 10�5

Pre-exponential factor for

steam reforming k1

9.95 � 1012

Pre-exponential factor for

reverse wateregas shift k2

1.65 � 1013

Pre-exponential factor

for decomposition reaction k3

1.65 � 1013

Universal gas constant R

(J mol�1 K�1)8314

Fluid phase thermal

conductivity kf(W m�1 K�1) [20]

0.04

Solid medium thermal

conductivity ks(W m�1 K�1) [20]

0.3

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 en e r g y 3 8 ( 2 0 1 3 ) 1 3 2 1 6e1 3 2 2 9 13219

reactions [33,34]. Therefore the chemical reactions taking

place during methanol steam reforming are.

Reaction 1: steam reforming reaction

CH3OHþH2O �����! �����k1

k�13H2 þ CO2 (11)

Reaction 2: reverse wateregas shift reaction

CO2 þH2 �����! �����k2

k�2COþH2O (12)

Reaction 3: decomposition reaction

CH3OH/k3COþ 2H2 (13)

Steam reforming reaction and reverse wateregas shift re-

action are reversible reactions. Decomposition reaction is a

non-reversible reaction. The constants k1, k2 and k3 are for-

ward rate constants and constants k�1, k�2 are backward rate

constants.

To simplify the analysis, the model proposed by Mastalir

et al. [35] has been used formethanol steam reforming and the

Arrhenius equation is used to calculate the reactant gases

generated by the chemical reaction.

rR ¼ k1C0:6CH3OHC

0:4H2O

exp

��Ea

RT

� k�1CCO2

CH2exp

��Ea

RT

(14)

rWGS ¼ k2CCO2CH2

exp

��Ea

RT

� k�2CCOCH2Oexp

��Ea

RT

(15)

rD ¼ k3C1:3CH3OHexp

��Ea

RT

(16)

2.4. Boundary conditions

At the inlet of the channel, the inlet flow velocity, gas

composition, steam to carbon ratio and temperature are

specified. The pressure at outlet is assumed to be equal to

atmospheric pressure. The boundary condition for the inter-

face between the flow channel and the catalyst layer is that

the velocities, temperature, species concentration and species

fluxes are continuous. On the interface between the catalyst

layer and substrate of the microreformer plate, the tempera-

ture is continuous and the normal velocity is zero, since there

is no flow across the solid boundary. Computations are per-

formed in one half of the flow channel due to symmetry.

Therefore, at the centreline of the channel ( y ¼ 0), the veloc-

ities, temperature and concentration gradients are zero. At the

wall ( y ¼ H/2) the velocities and concentration gradients are

assumed to be zero. A prescribed heat flux condition is

employed at the wall instead of isothermal wall condition as it

is more suitable from practical viewpoint.

The governing equations are solved numerically using a

general purpose CFD code Fluent 6.3which is a finite-volume

based code and the pressureevelocity coupling is attained

by using the SIMPLE algorithm. The solution was considered

to be converged when the residuals of all governing equations

approached steady state. The convergence criterion is that the

scaled residual variations of the mass, momentum and spe-

cies conservation equations become less than 1 � 10�6. Themicroreformer geometric parameters, various flow conditions

and the kinetic parameters used in this study are listed in

Table 2.

The performance of themicroreformer formethanol steam

reformation is evaluated in terms of conversion of methanol

and the concentration of carbonmonoxide (CO) in the product

gases which consists of mixture of H2, CO2 and CO at the

reformer outlet. The conversion rate of methanol for the

microreformer rc is calculated as,

rc ¼ Cm; 1 � Cm; 2

Cm; 1� 100 (17)

where Cm,1 and Cm,2 are the inlet and outlet molar concen-

trations of methanol, and the subscript m represents meth-

anol. Hydrogen yield is calculated from the following relation

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 2 1 6e1 3 2 2 913220

H2; yield ¼ CH2 ; 2

CH ; stoich� 100 (18)

2

CH2 ;yield is the molar concentration of hydrogen in the stoi-

chiometric steam reforming reaction of methanol (Reaction

(1)).

Fig. 2 e Comparison between predicted methanol

conversion and the experimental data of Chein et al. [36].

2.5. Grid details

A refined grid is necessary in the regionwhere the gradients of

the dependent variable are noticeable and accuracy of the

numerical solution depends on the grid size. Therefore, in the

computations, uniform mesh in the preliminary tests and

non-uniform finer mesh with more grids clustered in the re-

action region, near the wall and in the regions around the

fluidecatalyst layer interface were used in the succeeding

runs. Grid adaptation has been implemented to ensure good

resolution of the gradients of velocity, temperature and spe-

cies concentration in the computational domain. The

computational mesh is initialized with 90 � 35 (w3000 grid

points) grid elements in streamwise and cross-direction. The

initial mesh is adapted and refined for the period of a calcu-

lation to increase the accuracy of the solution in the regions of

high gradients. To evaluate the effect of grid size on the ac-

curacy of numerical solutions, the grid size was refined until

acceptable differences between the last two grid sizes were

found. After mesh refinement, a total of 3000e2,00,000 grid

elements are used and the numerical results show that the

solutions become grid independent when the number of grid

points is more than 40,000.

Fig. 3 e Effect of wall temperature and feed flow rate on the

mole fraction of carbon monoxide.

3. Results and discussion

3.1. Numerical model validation

To validate the present computational model, the preliminary

predictions for a plain channel have been carried out and the

results are compared with the corresponding experimental

data. The comparisons between the present predictions and

the experimental data provided by Chein et al. [36] for a plain

channel type methanol microreformer at various inlet feed

rates and reaction temperatures are shown in Fig. 2. The solid

symbols denote the experimental results and the curve rep-

resents the predictions. It is seen from the figure that higher

methanol conversion is obtained for lower feed rates. The

methanol conversion increases with an increase in the reac-

tion temperature (wall temperature). This is considered be-

tween 220 and 270 �C for comparison with the experimental

data. For a reaction temperature of 270 �C, g ¼ 1.1 and feed

flow rate of 2 ml h�1, the methanol conversion may achieve

76%. However, it is less than 40% for a reaction temperature of

220 �C and feed flow rate of 5 ml h�1. The conditions of this

benchmark case shown in this figure are same to that of Chein

et al. [36]. It is clear from Fig. 2 that the numerical results

match well with the experimental data for the range of

operating conditions. The effects ofwall temperature and feed

flow rate on the mole fraction of CO for the same case are

shown in Fig. 3. When the flow rate is increased, the mole

fraction of CO is significantly reduced. However, the trend of

dependence of COmole fraction onwall temperature and feed

rate is same as that of the methanol conversion.

3.2. Flow field analysis

To study the effect of cavities in the microreformer geometry

with details shown in Table 1, flow field, pressure drop and

cavity spacing effects are considered. The dimensionless ve-

locity profile along themidline of a single cavity in a channel is

shown in Fig. 4. Dimensionless velocity is defined as the ratio

of local flow velocity to uniform inlet velocity. In cavity area,

pressure increases because the slip velocity decreases. Ve-

locity in the cavities has some negative magnitude which is

relatively small compared to the main velocity component

due to flow entrapment in the cavities. For wall coated

microreformers this will reduce the acceleration caused by

the expansion due to heating of the gas mixture.

Flow rates required in the microreformer channel for a

given pressure drop will be higher if the number of cavities

increases. The influence of cavity spacing on pressure drop

Fig. 5 e Effect of mass flow rate and cavity spacing on the

pressure drop.

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 en e r g y 3 8 ( 2 0 1 3 ) 1 3 2 1 6e1 3 2 2 9 13221

(for equal relative cavity depth d* ¼ 2.5) in the channel is

shown in Fig. 5. As seen from the figure, an increase in cavity

spacing reduces the pressure drop. This can be attributed to

the fact that there is greater distortion of streamlines for the

channel with smaller cavity spacing and streamline distortion

is recovered for the channel with larger cavity spacing. For

wall coated reformers, most of the pressure drop occurs in the

porous catalyst support and pressure drop increases with in-

crease in flow rate. For reformers with cavities, the undesir-

able pressure drop is less as there is self-structured motion of

the fluid in the cavities. It is found that for a particular pres-

sure drop, the mass flow rates are approximately 21%, 18%

and 16% less than that of a smooth channel with relative

cavity depths of d* ¼ 2.5, 2 and 1 respectively, as shown in

Fig. 5. It is known that methanol conversion reduces with

higher feed flow rates (Fig. 2). This can be attributed to the fact

that for higher flow rates, the catalyst cannot supply adequate

active sites to methanol steam reforming reaction and the gas

mixture does not have enough time to diffuse from the gas

phase to the catalyst surface. Therefore, the reduced flow rate

in the channel with cavities and improvement in the resi-

dence time are expected to help in improving the operating

range of the reformer for higher feed rates and gas phase

mixing for the reforming reaction.

In the catalyst region, the flow has a lower velocity that

provides enough residence time for reactants on the catalyst

layer. Flow residence time is an important parameter for

methanol reforming and it indicates the contact time of the

reacting gases on the surface of catalysts. Chein et al. [36] have

reported that an average residence time of 0.5e0.7 s is enough

for reformation of methanol. The mean residence time is

calculated as

tm ¼ Lcux

(19)

where Lc is the length of the reforming catalyst bed and ux is

the average gas flow velocity. The residence time was calcu-

lated as 2.17 s at feed flow rate of 60 ml h�1.

Flow streamlines pattern and pressure contours for chan-

nels with different relative cavity depths and spacing are

Fig. 4 e Non-dimensional velocity variati

shown in Fig. 6. As seen from Fig. 6, the presence of cavities

noticeably perturbs the local flow near the channel wall. For a

channel with large cavity spacing (Fig. 6(a)), there is almost no

recirculating region in the cavities. As the spacing between

the cavities is reduced, the recirculation zone starts appearing

in the cavities and grows for larger cavity depths (Fig. 6(b)).

Fig. 6(c) shows that the pressure has a sharp drop near the

cavity entrance. Along the surface of the channel, the pressure

gradient is not monotonically negative like a plain channel

and it has some local variations due to presence of cavities. At

the inlet of the channel, there is a boundary layer and the flow

is not fully developed. Consequently this pressure variation

and local expansion of the fluid lead to the formation of

recirculation zones. There is sudden expansion and contrac-

tion in this area of the channel. The gas is compressed in the

narrower space and expanded in the wider space. This in-

dicates that the cavities exert a notable surface friction effect

on the gas flow field and enhance the lateral mixing by dis-

rupting the shear layer and this is expected to significantly

improve the methanol conversion rates. Such an effect also

on along the midline of the cavities.

Fig. 6 e (a) Streamline contours for channels with cavities (d* [ 2, s [ 5.62H ), (b) streamline contours for channels with

cavities (d*[ 2, s[ 1.87H ), (c) streamline contours for channels with cavities (d*[ 2.5, s[ 1.87H ) and (d) pressure contours

for channels with cavities (d* [ 2, s [ 5.62H ).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 2 1 6e1 3 2 2 913222

enhances the heat transfer characteristics to the advantage of

the reformer.

3.3. Steam-to-methanol molar ratio effect

The methanol conversion rate and CO concentration for

various steam-to-methanol molar ratios, g for a reforming

temperature of 250 �C and fixed flow rate of 50 ml h�1 are

shown in Fig. 7. As seen in the figure, a large steam-to-

methanol molar ratio is responsible for higher methanol

conversion as well as lower CO concentration. On the other

hand the hydrogen yield decreases with increase in steam-to-

methanol molar ratio. Higher steam in the gas mixture will

result in dilution of the hydrogen produced and will reduce

the hydrogen content in the reformed gas.

For steam-to-methanolmolar ratio g¼ 1.1, a conversion rate

ofmore than90%isachievedalongwithcertainamountofCOin

the reformed gas. Although, higher values of g result in higher

methanol conversion and reasonably lower content of CO, a

higher value of g implies higher heat flux requirements at the

walls of the reformer to vaporise the additionalwater present in

the inlet feedmixture.Therefore, an indicativevalueofg¼ 1.1 is

used for detailed studies in the analysis considering amethanol

conversion rate greater than 90% and lower heat input re-

quirements for endothermic steam reforming reaction.

3.4. Reformer GHSV effect

Fig. 8 shows the effect of reformer gas hourly space velocity

(GHSV) on the performance of the microreformer in terms of

methanol conversion rate, hydrogen yield and CO mole frac-

tion. GHSV is defined as volumetric flow rate per space vol-

ume. It is one of the key factors that have an effect on

methanol conversion because it is associated with the reac-

tion residence time inside the microreformer. It is seen that

hydrogen yield is maximum at a GHSV of 1388 h�1. The

hydrogen yield and the amount of CO decrease after this value

of GHSV. This is perhaps due to a decrease in maximum

temperature in the reformer for higher reformer GHSV. The

CO amount at this value of GHSV is about 0.08%. This lower

concentration of carbon monoxide in the gas stream elimi-

nates the need of water gas shift reactor equipment down-

stream in the fuel processing system for PEMFC containing

methanol steam microreformer. Therefore, reformer GHSV of

1388 h�1 in the channel is used as themost favorable value for

optimum performance of the reformer in this study.

Fig. 7 e Effect of steam-to-methanol molar ratio g on the

methanol conversion, hydrogen yield and CO

concentration.

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 en e r g y 3 8 ( 2 0 1 3 ) 1 3 2 1 6e1 3 2 2 9 13223

3.5. Point of reference results

Fig. 9(a) shows the temperature profiles at four different sec-

tions along the channel length for channel geometry consid-

ered in this study with heat flux sufficient to maintain the

channel walls at 250 �C and for parameters listed in Table 2.

Fig. 9(b) shows the centreline temperature variation in the

reformer and reaction rate along the channel length. As seen

from Fig. 9(b), the centreline temperature increases along the

reformer length due to heat flux received from the wall. The

temperature gradient across channel section is almost zero

after twenty percent of the channel length from inlet. This is

due to the fact that heat is absorbed by the reaction process.

This uniform temperature distribution along the reformer will

help in increased conversion of methanol and lower concen-

tration of CO. Near the channel inlet, the temperature in-

creases sharply andmethanol reforming reaction is active. As

Fig. 8 e Effect of GHSV in the channel on methanol

conversion, hydrogen yield and amount of CO.

seen from Fig. 9(b), the rate of reaction of the steammethanol

reaction along the channel length reaches a maximum value

in the vicinity of maximum reaction temperature and de-

creases rapidly in the downstream.

For the same operating condition of reforming temperature

of 250 �C and g ¼ 1.1, Fig. 10 shows the local distribution of

different species along the centreline of the channel. It is seen

that both themole fractions of the CH3OH andH2O decrease as

the fluid moves downstream, while the H2, CO2 and CO mole

fractions increase along the axial direction. CO concentration

is lower at inlet because the endothermic steam reforming

reaction is just initiated. As the endothermic reaction is nearly

completed, CO concentration becomes high near outlet. The

methanol conversion is about 98% for this reaction tempera-

ture and product gases consist of 75% H2, 23% CO2 and

928 ppm CO concentration at the outlet of the channel. Since,

the outlet gas carbonmonoxide concentration is less than 2%,

the clean-up step using water gas shift reactors can be elim-

inated and overall size of the fuel processing system for

PEMFC can be reduced. The decrease in H2O concentration

and increase in CO concentration along the channel length

indicate the dominance of methanol decomposition reaction

over steam reforming reaction along the axial direction.

The axial distribution of temperature at the centreline of

the catalyst layer and near the wall is shown in Fig. 11. The

comparison is made between the plain channel with uniform

layer of catalyst and a channel with cavities, d* ¼ 2.5 and

s ¼ 1.87H. The other operating parameters are reforming

temperature 250 �C, g ¼ 1.1 and GHSV of 1388 h�1. As seen

Fig. 9 e (a) Temperature profile and (b) variation of

centreline temperature and reaction rate along the channel

length.

Fig. 10 e Mole fraction variation of species along the channel.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 2 1 6e1 3 2 2 913224

from Fig. 11, for plain channel without cavities, the tempera-

ture at the centreline decreases in the first half of the catalyst

length due to endothermic steam reforming reaction and in-

creases in the downstream as the reaction rate decreases. In

case of channel with cavities, temperature is lower in the

initial part due to gas mixing and heat transfer augmentation

at the front edges of each catalyst section and reforming re-

action proceeds downstream in the catalyst layer. Therefore

steam reforming reaction is boosted through the augmenta-

tion of mass transfer at the front edges of the catalyst

sections.

Fig. 11 e Axial temperature distribution at the centre and

near the wall of the catalyst layer.

3.6. Effect of different cavity configurations

With an aim of studying the effect of different cavity config-

urations on methanol conversion and hydrogen yield, chan-

nels with different relative cavity depths are considered.

Relative cavity depths of d* ¼ 0, 1 and 2.5 are considered for

s ¼ 2.81H, GHSV of 1388 h�1, g ¼ 1.1 and reforming tempera-

ture of 250 �C for comparison. Fig. 12 shows the distribution of

methanol mole fraction and hydrogenmole fraction along the

centreline of the channels for different values of d*. As seen

from the figure, cavity depth has significant effect on local

methanol mole fractions. Methanol conversion takes place on

the sites of catalyst deposited on the fins. There is a drop of

13% in methanol conversion for the case of zero cavity depth

case. This case is equivalent to a plain channel case. There-

fore, methanol conversion decreases with a decrease in cavity

depths. Similarly some improvement over the local conver-

sion has been observed for d* ¼ 2.5 as compared to d* ¼ 1 case.

However, overall conversion rate remains same for both the

cases. The faster conversion can be attributed to the presence

of cavities in the reformer channel. Due to these faster re-

actions, the length of the channel is reduced by 20% as

compared to a plain channel for similar conversion. Higher

cavity depth hinders heat conduction in streamwise direction

and promotes surface reactions. Similarly, higher H2 mole

fraction along the channel signifies a higher methanol con-

version which is clearly seen for the case with higher cavity

depths. This increase ismainly due to increasedmass transfer

to the cavity walls for higher cavity depth channels compared

to smaller one that improves availability of reactants on re-

action sites. Overall, the higher depth of cavities results in

improved conversion rate for a given length of the channel.

The effect of cavity spacing ‘s’ onmethanol conversion rate

along the channel is shown in Fig. 13 for d*¼ 2.5, fixed channel

GHSV of 1388 h�1, g¼ 1.1 and various reforming temperatures.

It is seen from the figure that the cavity spacing has significant

effect onmethanol conversion. The results also show that the

methanol steam reformation reaction is strongly temperature

dependent. It is noticed that the predicted methanol conver-

sion increases along the microchannel length and decreases

with lower cavity spacing for all the reforming temperatures.

For lower cavity spacing, the catalyst deposition area is not

sufficient to provide active sites for methanol reformation.

Lower cavity spacing may obstruct the heat conduction

through the wall particularly for low reforming temperature

and thus will reduce the wall temperature. Therefore, chan-

nels with cavities improve the trade-off association towards

higher conversion rate at lower reforming temperatures

which results in lower CO concentration.

Fig. 12 e Effect of relative cavity depth on local CH3OH and H2 mole fraction along the channel.

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 en e r g y 3 8 ( 2 0 1 3 ) 1 3 2 1 6e1 3 2 2 9 13225

3.7. Heat transfer effects

Heat transfer in the channel with cavities is enhanced due to

intermittent interruptions of thermal boundary layers, divi-

sion of the bulk flow, and creation of the recirculating flow

structures inside the cavitieswhich destabilize the transversal

vortices in the cavities. There is also an improvement in the

local Nusselt number compared to smooth channels due to

distortion of thermal boundary layers and formation of recir-

culating flows inside the cavities. The heat transfer coefficient

h is definedas the ratioofwall heatfluxsupplied to thewall and

difference between wall temperature and bulk fluid tempera-

ture Tm. The bulk fluid temperature Tm is defined as,

Tm ¼

ZAc

uTdAc

Acu(20)

u is the average fluid velocity at a given cross-section location.

As seen from Fig. 13, the methanol steam reforming directly

depends on the wall temperature and hence depends on the

heat transfer characteristics. Fig. 14 shows the variation of

Nusselt number for various Reynolds numbers with s ¼ 2.81H,

Fig. 13 e Effect of cavity spacing on methanol conversion

rate along the channel for various reforming temperatures.

GHSV of 1388 h�1, g¼ 1.1 and reforming temperature of 250 �C.

It is seen from the figure that there is an increase in Nusselt

number as the Reynolds number increases. This is due to

formation of stronger recirculating zones in the cavities for

channels with higher relative cavity depth and reduction in

thermal boundary layer thickness. When relative cavity depth

becomes zero, a case of plain channel, the heat transfer is

relatively very small compared with a channel with cavities.

The effect of Reynolds number on the local distribution of

the methanol mole fraction along the channel length for

s¼ 2.81H, d*¼ 2.5, g¼ 1.1 and reforming temperature of 250 �Cis shown in Fig. 15. It is seen that better methanol conversion

rate is obtained for lower Reynolds number due to longer

residence time available for methanol conversion. Both GHSV

and Re will change with flow rate. Further, Re will change for

different characteristic dimensions of the channel (different

cavity depths). Therefore, both of these parameters are ex-

pected to have similar effects to that of change in the flow

rate. The hydrogenmole fraction for a fixed value of Reynolds

number and different inlet fluid temperatures shows that it

increases with an increase in inlet fluid temperature.

3.8. Effect of wall conductivity

The thermal conductivity of the wall material will affect the

heat transfer from wall to the reactants. Higher thermal

conductivity of the channel walls helps in increased heat

transfer from post-reformed region to preheat the unre-

formed gas in the upstream region. Radial heat conduction is

also helped by increasing the heat transfer from external

heating source to the fluid in the channel. To assess the

impact of wall thermal conductivity, three materials stainless

steel, brass and silicon with thermal conductivities of 16, 109

and 148 W m�1 K�1 respectively are considered. A high tem-

perature gradient prevails on the walls of lower thermal

conductivity material and this may result in the degradation

of the catalyst. Fig. 16 shows the temperature distribution and

methanolmole fraction along the centerline of the channel for

different wall materials with s ¼ 2.81H, GHSV of 1388 h�1,g ¼ 1.1 and reforming temperature of 250 �C. As seen in the

Fig. 14 e Variation of Nusselt number with Reynolds

number.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 2 1 6e1 3 2 2 913226

figure, the temperature distribution is muchmore uniform for

higher thermal conductivity material. The uniform tempera-

ture distribution improves the chemical reaction rate. There-

fore, methanol conversion for the microreformer with higher

thermal conductivity material is relatively much faster than

that with lower thermal conductivity material.

3.9. Effect of wall thickness

Thewall thickness plays an important role in the heat transfer

and thus in endothermic steam reforming reaction. The effect

of wall thickness onmethanol conversion rate, hydrogen yield

andmaximum temperature in the channel is shown in Fig. 17.

It is observed that when wall thickness is doubled, the

methanol conversion rate is reduced by 7% and hydrogen

yield decreases by 3%. The temperature distribution in the

Fig. 15 e Effect of Reynolds number on methanol mole fraction

centreline of the channel.

channel is more uniform for small channel wall thickness and

therefore better conversion is obtained.

3.10. Effect of catalyst layer thickness

The thickness of the catalyst layer plays an important role in

methanol conversion and CO concentration from the micro-

reformer. Four cases of catalyst layer thickness, 20, 30, 50 and

100 mm are studied to substantiate the effect of catalyst

thickness on heat transfer and performance of the microre-

former. Larger amount of gas mixture flows through central

part the channel in case of channel having higher catalyst

thickness. The velocity inside the catalyst layer will be less

and heat transfer will be dominated by conduction. The

temperature difference between the channel wall and bulk

fluid for various catalyst layer thickness considered here is

shown in Fig. 18. As discussed earlier, the temperature vari-

ation along the channel length is nonlinear and due to

chemical reaction, the flow is not fully thermally developed.

The smaller value of temperature difference between the

channel wall and the bulk fluid shown in Fig. 18 indicates that

bulk fluid temperature is closer to thewall temperature and as

a result corresponding Nusselt number values are on the

higher side.

Fig. 19 shows the methanol conversion and CO mole frac-

tion for various catalyst layer thicknesses considered here

with s ¼ 2.81H, GHSV of 1388 h�1, g ¼ 1.1 and reforming

temperature of 250 �C. As seen in Fig. 19, methanol conversion

rate increases linearly with catalyst thickness and both

methanol conversion rate and CO concentration are highest

for maximum catalyst thickness. Therefore, reasonable value

of catalyst layer thickness should be selected considering

practical issues related to catalyst layer coating on the

microreformer walls and conversion can further be improved

by selecting proper values of GHSV and reformer temperature.

The CO amount for 30 mm catalyst thickness is 0.08% which is

an acceptable level. Methanol steam reforming is a

and inlet temperature on hydrogen mole fraction along the

Fig. 18 e Temperature difference between channel wall

and bulk fluid and Nusselt number for various catalyst

layer thicknesses.

Fig. 16 e (a) Temperature profile and (b) methanol

conversion along the centreline of the channel for different

thermal conductivity materials.

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 en e r g y 3 8 ( 2 0 1 3 ) 1 3 2 1 6e1 3 2 2 9 13227

temperature governing chemical reaction and more uniform

temperature distribution is found for higher catalyst layer

thickness. Therefore, the reaction rate is higher for higher

catalyst layer thickness compared with the lower one and

better conversion and hydrogen yield is obtained.

Fig. 17 e Effect of wall thickness on methanol conversion,

hydrogen yield and maximum temperature in the channel.

4. Conclusion

Due to superior heat transfer andmass transfer performance of

the cavity type microreformers and high activity of the

commercially available catalyst, the methanol steam reform-

ing reaction can be completed more efficiently compared with

the conventional plain channel type microreformers. The

present study reports the detailed numerical analysis charac-

terizing the reforming reaction in a cavity type single channel

methanol microreformer for hydrogen production. The

microreformer is formed by two plates with integral cavities

placed one above the other and the commercial available Cu/

ZnO/Al2O3 catalyst is deposited on the fins of the two plates.

The methanol steam reforming reaction consisting of three

overall reactions is successfully applied. The predicted results

were successfully validated with the experimental data avail-

able in the literature. Various parameters such as wall con-

ductivity, thickness and catalyst layer thickness have a

Fig. 19 e Methanol conversion rate and CO concentration

for various catalyst thicknesses.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 2 1 6e1 3 2 2 913228

noteworthy impact on themethanol conversion rate. Effects of

different cavity parameters such as spacing and relative cavity

depth on the flow field and heat transfer characteristics,

methanol conversion rate and hydrogen production were

investigated. The trade-off association between conversion

rate and CO concentration was found with channel GHSV of

1388 h�1, steam-to-methanol molar ratio of 1.1, catalyst layer

thickness of 30 mmand reforming temperature of 250 �C for the

single channel plate type microreformer with cavities in this

study. Cavities perturb the local flow near the channel wall

resulting in the formation of recirculation zones leading to

increased heat and mass transfer and finally better methanol

conversion. Operating range of the reformer is increased as

high feed rate of inlet mixture reduces the conversion in plain

channel microreformers. Cavities help in reducing the flow

velocity inside the channels for the same feed flow rate. The

chemical reaction for methanol steam reforming is principally

associated with the wall and depends on the heat transfer

characteristics. The channel length can be reduced by 20%

compared to a plain channel for similar conversion rate.

Methanol conversion for the microreformer with higher ther-

mal conductivity material was relatively much faster than that

with lower thermal conductivity material and better conver-

sionwas observed for smaller wall thickness. This analysis can

be extended to design ofmicroreformers with inserted catalyst

layer which is potentially attractive option in practical appli-

cations because the catalyst layer is replaceable.

Acknowledgment

The financial support for this research from DST, Govt of

India, New Delhi is gratefully acknowledged.

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Glossary

Ci: molar concentration of species i, mol m�3

cp: specific heat, kJ kg�1 K�1

D: mass diffusion coefficient, m2 s�1

d: cavity depth, md*: relative cavitydepth (cavitydepthand channel height ratio, d/H )Ea: activation energy, J mol�1

H: channel height, mh: heat transfer coefficient, W m�2 K�1

DHD: heat of decomposition reaction, J mol�1

DHR: heat of reforming reaction, J mol�1

DHWGS: heat of reverse wateregas-shift reaction, J mol�1

keff: effective thermal conductivity, W m�1 K�1

kf: fluid phase thermal conductivity, W m�1 K�1

kp: catalyst layer permeability, m2

ks: solid medium thermal conductivity, W m�1 K�1

k1: pre-exponential factor for steam reforming reactionk2: pre-exponential factor for reverse wateregas-shift reactionk3: pre-exponential factor for decomposition reactionKn: Knudsen numberL: channel length, mLc: length of reforming catalyst bedl: characteristic geometric dimension, mMi: molecular weight of species i, kg mol�1

p: pressure, Pa

_Q: flow rate of the entering liquid mixture, cm3 h�1

R: universal gas constant, J mol�1 K�1

rc: methanol conversion raterR: Arrhenius reaction rate coefficient for steam reforming,

mol m�3 s�1

rWGS: Arrhenius reaction rate coefficient for reverse wateregas-shift reaction, mol m�3 s�1

rD: Arrhenius reaction rate coefficient for decomposition reaction,mol m�3 s�1

Si: source term of the pressure drop within porous medias: cavity spacing, mT: temperature, �Ctm: mean residence time, su, v: velocity components in x and y directions, respectively, m s�1

V: velocity, m s�1

Wt: flow channel wall thickness, mw: cavity width, mXi: mole fraction of species ix, y: coordinates, mYi: mass fraction of species i

Greek symbols

dC: catalyst layer thickness, mε: catalyst layer porosity4ij: ChapmaneEnskog parameterg: H2O/CH3OH molar ratiol: molecular mean free path, mm: dynamic viscosity, N m�2

r: density, kg m�3

yi: stoichiometric coefficients for reaction iyj: stoichiometric coefficients for product j

Subscripts

eff: effective1: inlet2: outletstoich: stoichiometry