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Journal ArticleDOI

2D heat and mass transfer modeling of methane steam reforming for hydrogen production in a compact reformer

01 Jan 2013-Energy Conversion and Management (Pergamon)-Vol. 65, pp 155-163

AbstractCompact reformers (CRs) are promising devices for efficient fuel processing. In CRs, a thin solid plate is sandwiched between two catalyst layers to enable efficient heat transfer from combustion duct to the reforming duct for fuel processing. In this study, a 2D heat and mass transfer model is developed to investigate the fundamental transport phenomenon and chemical reaction kinetics in a CR for hydrogen production by methane steam reforming (MSR). Both MSR reaction and water gas shift reaction (WGSR) are considered in the numerical model. Parametric simulations are performed to examine the effects of various structural/operating parameters, such as pore size, permeability, gas velocity, temperature, and rate of heat supply on the reformer performance. It is found that the reaction rates of MSR and WGSR are the highest at the inlet but decrease significantly along the reformer. Increasing the operating temperature raises the reaction rates at the inlet but shows very small influence in the downstream. For comparison, increasing the rate of heat supply raises the reaction rates in the downstream due to increased temperature. A high gas velocity and permeability facilitates gas transport in the porous structure thus enhances reaction rates in the downstream of the reformer.

Topics: Methane reformer (66%), Steam reforming (65%), Small stationary reformer (64%), Hydrogen production (57%), Reaction rate (57%)

Summary (3 min read)

1. Introduction

  • Hydrogen is an ideal energy carrier to support sustainable energy development [1].
  • In the long term, hydrogen can be produced in a clean way by solar thermochemical water splitting, photocatalytic water splitting or water electrolysis driven by solar cells/wind turbines [2,3].
  • In MSR reaction (Eq.1), methane molecules react with steam molecules to produce hydrogen and carbon monoxide in the catalyst layer of reformers.
  • It’s still not very clear how the change in inlet temperature and rate of heat supply can influence the coupled transport and reaction kinetics in the reformer, which are important for optimization of the reformer operation conditions.
  • As the present study do not consider the carbon deposition behavior in the reformer, a constant SCR of 2.0 is adopted.

2. Model development

  • Heat from the combustion duct is supplied to the Ni-based (i.e. [10]) catalyst layer via the solid thin film layer and it is specified as a boundary condition [6].
  • Without considering the 3D effect, the coupled transport and chemical reaction phenomena in the computational domain can be shown in Figure 2, including the solid plate, the reforming duct, and the porous catalyst layer.
  • The chemical model is developed to calculate the rates of chemical reactions and corresponding reaction heats.
  • The CFD model is used to simulate the heat and mass transfer phenomena in the CR.

2.1 Chemical model

  • In operation, methane-containing gas mixture (CH4: 33%; H2O: 67%) is supplied to the reforming duct.
  • The gas species are then transported from the gas duct into the porous catalyst layer, where MSR reaction (Eq. 1) and WGSR (Eq. 2) take place.
  • The formulas proposed by Haberman and Young [11] have been widely used for simulating the rates (mol.m-3.s-1) of MSR ( MSRR ) and WGSR ( WGSRR ), thus is adopted in the present study.
  • The amount of heat generation from WGSR and heat consumption by MSR reaction can be calculated using corresponding enthalpy changes [12].
  • Assuming linear dependence on operating temperature between 600K and 1200K, the reaction heats (J.mol-1) for MSR reaction and WGSR can be calculated as [13].

2.2. Computational Fluid Dynamics (CFD) model

  • Assuming local thermal equilibrium in the porous catalyst layer, the governing equations for mass conservation, momentum conservation, and energy conservation for the whole computational domain are summarized below [14].
  • The Darcy’s law (Eq.26 and 27) is used as source terms in momentum equations (Eqs. (13) and (14)), so that the momentum equations are applicable for both the gas channels and the porous catalyst layers.
  • The source term in energy equation (Eq. (15)) represents reaction heat from the chemical reactions can be calculated by Eq. (28).
  • Detailed descriptions of the source terms can be found in the previous publications [17].

2.3 Numerical scheme

  • The governing equations in the CFD model are solved with the finite volume method (FVM) [14].
  • As a real reformer stack consists of many identical single compact reformers, it is assumed that heat is supplied from the combustion channel (Fig. 1) and there is no heat transfer between compact reformers through the upper boundary (y=yM).
  • The convection terms and diffusion terms are treated with the upwind difference scheme and central difference scheme, respectively.
  • The velocity and pressure are linked with the SIMPLEC algorithm.
  • Computation is repeated until convergence is achieved.

3. Results and discussions

  • The chemical model and CFD model have been validated in the previous publications by comparing the modeling results with data from the literature [17].
  • The dimensions and typical simulation parameters are summarized in Table 2.
  • The following sections focus on parametric simulations to analyze the effects of operating and structural parameters on the coupled transport and reaction kinetics in CR.

3.1 Coupled transport and reaction in a compact reformer for hydrogen production

  • Figure 3 shows the distributions of MSR reaction rates, WGSR rates, temperature, velocity, gas composition (CH4 and H2 as examples) in the compact reformer at an inlet temperature of 1073K, inlet gas velocity of 3m.s-1, and heat supply rate (from the solid plate) of 1kW.m-2.
  • The reaction rates for MSR and WGSR are the highest (25.4 and 14 mol.m-3.s-1 respectively) at the inlet and decrease considerably in the downstream of the reformer (Fig. 3a and 3b).
  • In addition, the temperature is the highest at the inlet (Fig. 3c).
  • A locally low molar fraction of CH4 is also observed near the inlet in the catalyst layer (Fig. 3e).
  • For comparison, the molar fraction of H2 increases along the CR gas flow stream (Fig. 3f).

3.2. Effect of inlet temperature

  • The reaction rates of MSR and WGSR are found to decrease along the main flow stream (Fig. 4a and 4b), but their values are significantly higher than those at 1073K (Fig. 3a and 3b).
  • In addition, the reaction rates decrease more rapidly in the reformer than at 1073K.
  • The high reaction rate of MSR causes the temperature to decrease rapidly along the main flow stream from 1173K at the inlet to about 1040K at the outlet (Fig. 4c).
  • As the reaction rates of MSR and WGSR are higher at 1173K than at 1073K, more CH4 is consumed and more H2 is produced, leading to larger gas composition variation in the reformer (Fig. 4d and 4e).
  • In a word, increasing the inlet temperature increases the reaction rates, temperature gradient, and gas composition variation.

3.4. Effect of inlet gas velocity and microstructure of the catalyst layer

  • It’s found that the reaction rates of MSR and WGSR are the highest at the inlet but decrease considerably along the reformer, due to large temperature drop along the main flow stream.
  • Three-dimensional simulation of chemically reacting gas flows in the porous support structure of an integrated-planar solid oxide fuel cell, Int. J. Heat Mass Transfer 47(2004) 3617-3629. [12].
  • Parameters used in calculating the effective diffusion coefficients [16].

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1
2D Heat and Mass Transfer Modeling of Methane Steam
Reforming for Hydrogen Production in a Compact Reformer
Meng Ni
Building Energy Research Group, Department of Building and Real Estate,
The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, P.R. China
Abstract:
Compact reformers (CRs) are promising devices for efficient fuel processing. In CRs, a thin solid
plate is sandwiched between two catalyst layers to enable efficient heat transfer from combustion
duct to the reforming duct for fuel processing. In this study, a 2D heat and mass transfer model is
developed to investigate the fundamental transport phenomenon and chemical reaction kinetics in a
CR for hydrogen production by methane steam reforming (MSR). Both MSR reaction and water
gas shift reaction (WGSR) are considered in the numerical model. Parametric simulations are
performed to examine the effects of various structural/operating parameters, such as porosity,
permeability, gas velocity, temperature, and rate of heat supply on the reformer performance. It is
found that the reaction rates of MSR and WGSR are the highest at the inlet but decrease
significantly along the reformer. Increasing the operating temperature raises the reaction rates at the
inlet but shows very small influence in the downstream. For comparison, increasing the rate of heat
supply raises the reaction rates in the downstream due to increased temperature. A high gas velocity
and permeability facilitates gas transport in the porous structure thus enhances reaction rates in the
downstream of the reformer.
Keywords:
Compact reformer; Fuel processing; Porous media; Heat and mass transfer; Hydrogen
production
* Corresponding author. Tel: (852) 2766 4152; Fax: (852) 2764 5131;
Email: meng.ni@polyu.edu.hk (Meng Ni)
This is the Pre-Published Version.

2
1. Introduction
Hydrogen is an ideal energy carrier to support sustainable energy development [1]. Using a
fuel cell, hydrogen can be efficiently converted into electricity with water as the by-product. To
make the hydrogen energy and fuel cell commercially feasible, it is critical to produce hydrogen
efficiently and economically at a large scale.
In the long term, hydrogen can be produced in a clean way by solar thermochemical water
splitting, photocatalytic water splitting or water electrolysis driven by solar cells/wind turbines [2,3].
However, the present energy efficiencies of both thermochemical and photocatalytic hydrogen
production methods are too low to be economically viable (i.e. efficiency for photocatalytic
hydrogen production is usually less than 1% [2]). Water electrolytic hydrogen production can be a
promising technology for large scale hydrogen production but the cost is still high, due to the use of
expensive catalyst, i.e. Pt. For comparison, steam reforming of hydrocarbon fuels (i.e. methane) is
efficient and can be a feasible way for hydrogen production for the near term [4]. In general,
hydrogen production from methane is based on one of the following processes: methane steam
reforming (MSR), partial oxidation (POX), and autothermal reforming (ATR) [5]. MSR is the most
common method for hydrogen production from methane at a large scale. In MSR reaction (Eq.1),
methane molecules react with steam molecules to produce hydrogen and carbon monoxide in the
catalyst layer of reformers. Meanwhile, steam can react with carbon monoxide to produce
additional hydrogen and carbon dioxide (Eq. 2), which is called water gas shift reaction (WGSR).
42 2
3CH H O CO H
(1)
222
CO H O CO H
(2)
WGSR is exothermic while MSR is highly endothermic. As the MSR reaction rate is
usually higher than WGSR, heat is required for hydrogen production by MSR and WGSR. The heat
supply can be achieved by using a compact reformer (CR). A typical CR consists of a solid thin

3
plate sandwiched between two catalyst layers, as can be seen from Figure 1 (adapted from [6]). The
small thickness of the thin plate allows efficient heat transfer from the combustion duct to the fuel
reforming duct to facilitate chemical reactions in the catalyst layer. High power density resulted
from the compactness nature of the CRs makes them suitable for stationary and transportation
applications [7,8]. Although some preliminary studies have been performed for CRs, there is
insufficient numerical modeling on CRs for hydrogen production by methane steam reforming,
especially on how the various parameters affect the reformer performance. It’s still not very clear
how the change in inlet temperature and rate of heat supply can influence the coupled transport and
reaction kinetics in the reformer, which are important for optimization of the reformer operation
conditions. In addition, the study in the literature considers pre-reformed methane gas consisting of
CH
4
, H
2
O, CO, CO
2
, and H
2
gas mixture at the inlet [6]. While it may be more appropriate to use
CH
4
/H
2
O mixture as the feeding gas to the reformer.
In this paper, 2D numerical model is developed to simulate the performance of a CR for
methane reforming. Different from the previous studies using pre-reformed gas mixtures at the
inlet, the present study uses a CH
4
/H
2
O mixture at the reformer inlet. In real application, the steam
to carbon ratio (SCR) is an important parameter as carbon deposition can occur at a low (i.e. less
than 1) SCR [9]. As the present study do not consider the carbon deposition behavior in the
reformer, a constant SCR of 2.0 is adopted. The effects of the reformer structural/operating
parameters on the coupled transport and reaction phenomena are investigated and discussed in
detail.
2. Model development
A 2D model is developed for hydrogen production from methane reforming in a CR. Heat
from the combustion duct is supplied to the Ni-based (i.e. [10]) catalyst layer via the solid thin film

4
layer and it is specified as a boundary condition [6]. Without considering the 3D effect, the coupled
transport and chemical reaction phenomena in the computational domain can be shown in Figure 2,
including the solid plate, the reforming duct, and the porous catalyst layer. The 2D model consists
of a chemical model and a CFD model. The chemical model is developed to calculate the rates of
chemical reactions and corresponding reaction heats. The CFD model is used to simulate the heat
and mass transfer phenomena in the CR.
2.1 Chemical model
In operation, methane-containing gas mixture (CH
4
: 33%; H
2
O: 67%) is supplied to the
reforming duct. The gas species are then transported from the gas duct into the porous catalyst
layer, where MSR reaction (Eq. 1) and WGSR (Eq. 2) take place. The formulas proposed by
Haberman and Young [11] have been widely used for simulating the rates (mol.m
-3
.s
-1
) of MSR
(
M
SR
R
) and WGSR (
WGSR
R
), thus is adopted in the present study.
2
42
3
CO H
MSR rf CH H O
ps
PP
RkPP
K





(3)
231266
2395exp
rf
k
RT



(4)
10 4 3 2
1.0267 10 exp 0.2513 0.3665 0.5810 27.134 3.277
pr
KZZZZ
(5)
22
2
CO H
WGSR sf CO H O
ps
PP
RkPP
K





(6)
103191
0.0171exp
sf
k
RT



(7)

32
exp 0.2935 0.6351 4.1788 0.3169
ps
KZZZ
(8)

5
1000
1
()
Z
TK
 (9)
where T is the temperature (K), R is the universal gas constant (8.3145 J.mol
-1
K
-1
). P is partial
pressures of gas species (Pa).
The amount of heat generation from WGSR and heat consumption by MSR reaction can be
calculated using corresponding enthalpy changes [12]. Assuming linear dependence on operating
temperature between 600K and 1200K, the reaction heats (J.mol
-1
) for MSR reaction and WGSR
can be calculated as [13].

206205.5 19.5175
MSR
H
T
(10)
45063 10.28
WGSR
H
T
(11)
2.2. Computational Fluid Dynamics (CFD) model
Assuming local thermal equilibrium in the porous catalyst layer, the governing equations for
mass conservation, momentum conservation, and energy conservation for the whole computational
domain are summarized below [14].

0
UV
xy




(12)

x
UU VU
PU U
S
xyxxxyy












(13)

y
UV VV
PV V
S
xyyxxyy












(14)

PP
T
cUT cVT
TT
kkS
xyxxyy











(15)

,,im im
ii
eff eff
ii
s
p
UY VY
YY
D
DS
xyxxyy













(16)

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Abstract: Nano-sized TiO 2 photocatalytic water-splitting technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy. Presently, the solar-to-hydrogen energy conversion efficiency is too low for the technology to be economically sound. The main barriers are the rapid recombination of photo-generated electron/hole pairs as well as backward reaction and the poor activation of TiO 2 by visible light. In response to these deficiencies, many investigators have been conducting research with an emphasis on effective remediation methods. Some investigators studied the effects of addition of sacrificial reagents and carbonate salts to prohibit rapid recombination of electron/hole pairs and backward reactions. Other research focused on the enhancement of photocatalysis by modification of TiO 2 by means of metal loading, metal ion doping, dye sensitization, composite semiconductor, anion doping and metal ion-implantation. This paper aims to review the up-to-date development of the above-mentioned technologies applied to TiO 2 photocatalytic hydrogen production. Based on the studies reported in the literature, metal ion-implantation and dye sensitization are very effective methods to extend the activating spectrum to the visible range. Therefore, they play an important role in the development of efficient photocatalytic hydrogen production.

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"2D heat and mass transfer modeling ..." refers background or methods in this paper

  • ...Both MSR reaction and water gas shift reaction (WGSR) are considered in the numerical model....

    [...]

  • ...Parametric simulations are performed to examine the effects of various structural/operating parameters, such as porosity, permeability, gas velocity, temperature, and rate of heat supply on the reformer performance....

    [...]


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01 Jan 1998

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Abstract: 3.8.2. Temperature Distribution Measurements 4749 3.8.3. Two-Phase Visualization 4750 3.8.4. Experimental Validation 4751 3.9. Modeling the Catalyst Layer at Pore Level 4751 3.10. Summary and Outlook 4752 4. Direct Methanol Fuel Cells 4753 4.1. Technical Challenges 4754 4.1.1. Methanol Oxidation Kinetics 4754 4.1.2. Methanol Crossover 4755 4.1.3. Water Management 4755 4.1.4. Heat Management 4756 4.2. DMFC Modeling 4756 4.2.1. Needs for Modeling 4756 4.2.2. DMFC Models 4756 4.3. Experimental Diagnostics 4757 4.4. Model Validation 4758 4.5. Summary and Outlook 4760 5. Solid Oxide Fuel Cells 4760 5.1. SOFC Models 4761 5.2. Summary and Outlook 4762 6. Closing Remarks 4763 7. Acknowledgments 4763 8. References 4763

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"2D heat and mass transfer modeling ..." refers methods in this paper

  • ...The CFD model is used to simulate the heat and mass transfer phenomena in the CR....

    [...]

  • ...206205.5 19.5175MSRH T (10) 45063 10.28WGSRH T (11)...

    [...]



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Frequently Asked Questions (2)
Q1. What contributions have the authors mentioned in the paper "2d heat and mass transfer modeling of methane steam reforming for hydrogen production in a compact reformer" ?

In this study, a 2D heat and mass transfer model is developed to investigate the fundamental transport phenomenon and chemical reaction kinetics in a CR for hydrogen production by methane steam reforming ( MSR ). 

The effects of SCR and the catalyst nature on CR performance are not included but will be considered in future works.