Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study

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Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study Junjie Chen*, Longfei Yan, Wenya Song, Deguang Xu Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo, Henan, China

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abstract

Article history:

This paper addresses the issues related to the design and operation of steam reforming

Received 19 April 2018

combined with catalytic combustion in thermally integrated microchannel reactors for

Accepted 1 June 2018

hydrogen production. Comparisons were made between methanol and methane steam

Available online xxx

reforming, representing a low and a high temperature process respectively, under the same operating conditions to determine whether methanol-based thermally integrated

Keywords:

systems can be more energy-efficient than methane-based ones. Computational fluid dy-

Microchannel reactors

namics simulations were performed to gain insight into the reactor performance and

Hydrogen production

thermal behavior. The effect of various design parameters was investigated to identify

Steam reforming

suitable ranges of operating conditions, and an analysis of heat and mass transfer was

Catalytic microreactors

performed to design a highly efficient system. It was shown that steam reforming of both

Micro-combustion

fuels is feasible in millisecond reactors under a variety of conditions, but very careful

Computational fluid dynamics

design is necessary. Methanol reforming can be more efficient, offering a better solution not only to simplify design but also to improve power and efficiency. The wall thermal conductivity is essential to the design and optimization of these systems, as it can significantly affect the overall energy balance. There is no significant difference in reactor performance between different channel heights at the same flow rate. The ratio of the flow rates on opposite sides of the reactor is an important design parameter and must be carefully adjusted to improve efficiency and eliminate hot spots. Finally, a simple operating strategy was proposed to achieve variable power output, and design recommendations were made. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction In recent years, there has been an increasing interest in the development of portable fuel processing systems for hydrogen

production [1e4]. Since conventional systems are often limited by physical transport effects, microreactor technology has the potential to greatly improve overall performance for fuel cell applications [4e12]. The major techniques used to produce hydrogen from hydrocarbon fuels are steam

* Corresponding author. Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, 2000 Century Avenue, Jiaozuo, Henan, 454000, PR China. E-mail addresses: [email protected], [email protected] (J. Chen). https://doi.org/10.1016/j.ijhydene.2018.06.001 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Chen J, et al., Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.001

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Nomenclature A Cfuel,in cp d dpore D Deff Dm DT F Fs-∞ h ho Dc Hqm ðTÞ

surface area initial concentration of the fuel specific heat capacity at constant pressure gap distance between the plates mean pore diameter diffusivity effective diffusivity mixture-averaged diffusivity thermal diffusivity catalyst/geometric surface area view factor for solid-ambient specific enthalpy overall heat transfer coefficient standard enthalpy of combustion at the specified temperature Dr Hqm ðTÞ standard enthalpy of reaction at the specified temperature K number of species l length m total number of species p pressure q heat flux Q_ molar flow rate R ideal gas constant s_ rate of appearance of a heterogeneous product Brunauer-Emmett-Teller surface area SBET S/V surface-to-volume ratio absolute temperature, reference temperature T, To temperature at the external surface of the solid Tw,o wall u, v streamwise velocity component, transverse velocity component catalyst pore volume Vpore ! V, V diffusion velocity, diffusion velocity vector relative molecular mass, relative molecular mass W, W of the gas mixture

reforming, partial oxidation, and autothermal reforming [13e16]. Steam reforming is typically the preferred process for hydrogen production in industry [3]. Due to the importance of this reaction, substantial efforts are being made to ensure stable and efficient operation of portable fuel processing systems [17e24]. Unfortunately, there exist a number of key challenges for the realization of these systems [5,25]. The management of heat within a highly compact device is generally regarded as the most crucial challenge [26,27]. Thermal coupling of exothermic and endothermic reactions is needed to reduce heat losses and improve thermal efficiency. Additionally, mass-transport effects are usually negligible in these systems and thus intrinsic kinetics tend to dominate, but in reality this is not always the case [5,28]. Furthermore, the overall performance of these portable fuel processing systems is highly dependent on the reactor dimensions [29,30] and operating conditions [18,31,32]. In particular, the dependence of kinetics

x, y Y

streamwise coordinate, transverse coordinate mass fraction

Greek variables G surface site density d thickness ε emissivity εm porosity effective emissivity for solid-ambient εs∞ h thermal efficiency hreforming thermal efficiency of the reforming process l thermal conductivity porous medium effective thermal conductivity lm leff effective thermal conductivity m dynamic viscosity n stoichiometric coefficient r density s Stefan-Boltzmann constant site occupancy of the m-th surface species sm t time-scale tortuosity factor tm u_ rate of appearance of a homogeneous product Subscripts (þ) just above the interface () just below the interface amb ambient c catalyst g gas in inlet i, j, k, m species index o outer rad radiation s solid w wall x, y streamwise component, transverse component

with temperature tends to be the dominant factor in design and operation, and must be clearly established. It is therefore of great significance to clarify the role of operating temperature in these portable fuel processing systems. The steam reforming along with water-gas shift reaction is kinetically limited at low temperatures but thermodynamically limited at high temperatures [33,34]. From the point of view of reduced carbon monoxide content and increased hydrogen production, it is desirable to conduct the reforming process at low temperatures [33]. However, to achieve the necessary reaction rates, higher temperatures are often required [34]. Unfortunately, higher-temperature processes require more insulation and thermal integration. An ideal condition would be the use of a catalyst active enough to operate at low temperatures where equilibrium is very favorable [33,34]. The operating temperature during the process of steam reforming is mainly determined by the fuel used. Alcohol fuels

Please cite this article in press as: Chen J, et al., Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.001

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and hydrocarbon fuels can both be used as fuels. The reaction takes place at essentially two different temperature ranges [35]. More specifically, the steam reforming of methanol takes place at substantially lower temperatures, typically within 250e300  C, whereas steam reforming of other fuels requires temperatures typically greater than 500  C. Typically, methane steam reforming often needs temperatures up to 900  C to achieve high equilibrium yields. For portable hydrogen production, liquid fuels such as alcohols are possible candidates. In particular, methanol is an attractive fuel, offering high energy density, high hydrogen-carbon ratio, and ready availability [13]. In addition, methanol has the advantages of being sulfur-free and ultra-clean [3,5]. Furthermore, low operating temperatures can minimize heat losses and reduce the undesired carbon monoxide [13], thereby offering a significant advantage over hydrocarbon steam reforming [14]. However, there are some drawbacks for methanol, such as a lack of infrastructure for fuel distribution and environmental concerns as well as a lower energy density [14,36]. In contrast, hydrocarbon fuels are more readily available, and they also offer other advantages such as higher energy density, thus making them attractive candidates for portable applications [3]. Compared to alcohol steam reforming, hydrocarbon steam reforming generally requires higher operating temperatures [37]. However, the necessity for hightemperature processing is a drawback for hydrocarbon steam reforming [3,14] due to the issues associated with thermal management and carbon formation. In particular, the high operating temperatures impose stringent requirements both on design and on defining operating conditions [26]. Furthermore, the water-gas shift reaction is often used in conjunction with steam reforming of hydrocarbon fuels to improve the yield of hydrogen and to effectively reduce the content of carbon monoxide [14]. Low-temperature alcohol reforming tend to be more energy-efficient than high-temperature hydrocarbon one. In contrast, hydrocarbon-air flames in small-scale combustion systems are more robust than alcohol-air ones; combustion stability can be improved and higher reaction temperatures can be achieved. Consequently, it is not clear whether lowtemperature alcohol reforming in thermally integrated microreactors could be more energy-efficient than hightemperature hydrocarbon reforming when coupled with their respective combustion, as well as how to design a highly efficient micro-system for short-contact-time operation. This paper addresses the issues related to the stable and efficient operation of combined combustion and reforming in thermally integrated microchannel reactors for the production of hydrogen. Such a design has received increasing attention, which is based on a catalytic plate heat exchanger structure so that highly exothermic and endothermic reactions can occur in alternate channels. This is known as a catalytic plate reactor or a catalytic wall reactor, which can simultaneously reduce heat and mass transfer limitations for reactions, thus making it potentially more efficient in fuel cell applications. Even further improvements in terms of heat and mass transfer can be achieved by microreactors [4], thereby offering clear advantages over conventional reactors. Scale-up microreactors can be achieved by configuring several

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microreactors in parallel. Multiple reactors can be further connected in parallel to achieve any desired large-scale plant capacity [4]. In order to determine whether low-temperature alcohol reforming in thermally integrated systems could be more energy-efficient than high-temperature hydrocarbon one, comparisons were made between them in terms of reactor performance. Methanol and methane steam reforming were chosen as a representative of a low-temperature and a hightemperature process, respectively. Numerical simulations were performed and an analysis of heat and mass transfer was made to provide guidance on how various design parameters affect the reactor performance. The objective of this paper is to evaluate the role of various design parameters so as to identify suitable ranges of operating conditions. Particular emphasis is placed on the comparisons in performance between the methanol and methane reforming processes in an effort to design a highly efficient system. This study can provide guidelines for developing an effective energy management strategy in the design of portable fuel processing systems.

Model development Detailed modeling is necessary to accurately describe the operation of these portable fuel processing systems [38,39] as well as to better understand transport phenomena occurring in these systems [8,20], although it is very complex due to the complex interplay between transport and kinetics in different phases and spatial directions [28].

Physical model A parallel-plate reactor having sub-millimeter flow channels is depicted in Fig. 1. The reactor is designed with adjacent channels of catalytic combustion and steam reforming, permitting high heat flux operation [19,20]. Five different regions can be identified: combustion and reforming channels, the dividing wall between them, and catalyst layers for combustion and reforming. The endothermic reforming reaction and the exothermic combustion reaction take place on opposite sides of the reactor, respectively. In the high temperature process, methane steam reforming over Rh/Al2O3 catalysts is thermally coupled with methane catalytic combustion over Pt/Al2O3 catalysts in adjacent channels. Rhodium is considered due to its very high conversion and excellent selectivity to syngas [40]. The low-temperature process is the steam reforming of methanol over Pd/ZnO/Al2O3 catalysts, with heat supplied by integrated catalytic combustion of methanol over CuO/ZnO/Al2O3 catalysts. Recent experiments have demonstrated that palladium supported on zinc oxide is non-pyrophoric and exhibits excellent thermal stability, thus offering an advantage for the portable production of hydrogen [22,41]. In order to facilitate the analysis and design, a “base case”, for which 99% conversion of fuel (i.e., the breakthrough limit, as will be defined later) is achieved in the reforming channel for both thermally integrated systems, is given in Table 1. This approach makes it convenient to compare different systems

Please cite this article in press as: Chen J, et al., Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.001

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Fig. 1 e Two-dimensional schematic diagram of the microchannel reactor in a parallel plate configuration. To minimize the computational intensity, only half of each channel and the connecting wall are modelled by taking properly into account the inherent symmetry of the geometry. The “symmetry planes” are the axes of symmetry in each channel. For the “base case”, the combustion channel is 0.8 mm high, the reforming channel is 0.8 mm high, and the separating wall is 0.2 mm thick. The arrows indicate directions of flow.

under the same operating conditions. In this context, the effect of various design variables can be conveniently evaluated further. For each of the systems, the channel dimensions are nominally 0.8 mm high and 50.0 mm long, and the wall separating the reforming and combustion side is 0.2 mm. On the combustion side, the equivalence ratios of a methane-air mixture and a methanol-air mixture are assumed to be 0.83

Table 1 e Model parameters used for the base case computation. For a “base case”, 99% conversion of fuel is achieved in the reforming channel for both thermally integrated systems. Combustion side Geometry Plate length Channel height Gas phase Inlet conditions Pressure Temperature Velocity (methane) Velocity (methanol) Equivalence ratio (methane) Equivalence ratio (methanol) Molar steam-to-carbon ratio Catalyst layer Thickness Catalyst/geometric surface area Porosity Tortuosity factor Solid wall Thickness Thermal conductivity

Reforming side

50.0 mm 0.8 mm

0.1 MPa 300 K 4.0 m/s 4.0 m/s 0.83 0.8

400 K 2.2 m/s 9.2 m/s

2.0 0.02 mm 30 0.5 3

0.6 3

0.2 mm 80 W/(m$K)

The characteristics of the catalysts listed above are obtained from the provided kinetic schemes. Note that the same catalyst/geometric surface area is used for these catalysts.

and 0.8, respectively, at the inlet. In this case, the overall amount of heat released during the complete combustion of the two fuels is the same, allowing a more meaningful comparison between the two systems. The choice of these equivalence ratios is also dictated by the operation space that satisfies the requirements of combustion stability, fuel conversion, and materials stability [42]. The optimal molar steam-to-carbon ratio is often higher than the stoichiometry, typically 3.0 or greater for methane steam reforming [3] and 1.1 for methanol steam reforming [7] to obtain the highest hydrogen yield. Note that the presence of atomic oxygen within the molecular structure of methanol makes it possible to use a lower-than-usual ratio, and enables the reforming process more convenient. In the present work, the molar steam-to-carbon ratio in the feed is assumed to be 2.0 on the reforming side for each of the two systems, since a direct comparison between them is made on the basis of the same operating conditions. More importantly, operation at lower steam-to-carbon ratios can increase the power output at lower reaction temperatures and can reduce the pressure drop, thus making methane steam reforming more energy efficient [43], although it is not typically pursued in practice since lower steam-to-carbon ratios may reduce the catalyst life [8]. Flow rates, whenever reported, assume a reactor width of 10.0 mm. Note that the width of the reactor refers to the third dimension (perpendicular to both the x and y directions), which is not shown in Fig. 1. There are many possible design configurations. The reactor is operated in a co-current flow arrangement as shown in Fig. 1, which can improve the heat exchange efficiency within the system and thus minimize hot spots [44,45]. In contrast, counter-current operation can yield higher conversion, which may be advantageous for some equilibrium limited reactions [46], but, unfortunately, such arrangement is not desirable in thermally integrated systems due to its pronounced temperature extremes [44]. Therefore, only co-current flow is considered in the present work.

Please cite this article in press as: Chen J, et al., Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.001

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    v vT v vT ls þ ls ¼ 0: vx vx vy vy

Mathematical model In this model, ideal gas behavior is assumed; steady state is considered; gas-phase radiative transfer is negligible [47]; and the catalyst layer is assumed to be isothermal in the transverse direction (i.e., the y direction shown in Fig. 1) because of its small thickness; homogeneous reactions are negligible due to their contributions only at relatively high temperatures [34,48]. Only in the case of methane-based system, the combustion reaction can simultaneously take place in the gas phase and on the surface of the catalyst [47]. Laminar flow is assumed in both channels, since the Reynolds numbers are less than 280. Radiative heat exchange between the surfaces is modeled on both sides of the reactor. A two-dimensional numerical model is developed using ANSYS Fluent® Release 16.0 [49]. The steady-state conservation equations are solved in the gas phase. Continuity equation: vðruÞ vðrvÞ þ ¼0 vx vy

(1)

Momentum equations:

(2)


  vðruvÞ vðrvvÞ vp v vv vu þ þ  m þ vx vy vy vx vx vy
  v vv 2 vu vv 2m  m þ ¼ 0:  vy vy 3 vx vy

(3)

þ Fcat=geo Wk ðs_k Þinterface ¼ 0; k ¼ 1; …; Kg :

(9)

Acatalyst : Ageometric

(10)

The emissivity of each element of the surfaces is assumed to be 0.8 [55]. The energy boundary condition at the specified gas-wall interfaces is written as    Kg X vT vT þ ls þ ðs_k hk Wk Þinterface ¼ 0: vy interface vy interfaceþ k¼1 (11)

(4)

While similar energy boundary conditions are employed at the left and right edges of the wall, these interfaces are chemically inert. The total heat loss from these interfaces to the surroundings is given by  q ¼ ho ðTw;o  Tamb Þ þ εs∞ Fs∞ s T4w;o  T4amb :

k ¼ 1; …; Kg ; (5) ! where the diffusion velocity, V k , can be computed by using the mixture-averaged diffusivity, including thermal diffusivity for the light species such as gas-phase atomic and molecular hydrogen [50]: (6)

The coverage equations of the surface species are given by s_m ¼ 0; m ¼ Kg þ 1; …; Kg þ Ks : G

interface



 vðruYk Þ vðrvYk Þ v v  þ þ ðrYk Vk;x Þ þ rYk Vk;y  u_ k Wk ¼ 0; vx vy vx vy

sm



where the catalyst/geometric surface area, Fcat=geo , is defined as [54]:

q_rad  lg

Gas phase species equation:

     

! V k ¼ Dk;m V ln Yk W Wk þ DTk W rYk W Vðln TÞ:

rYk Vk;y

Fcat=geo ¼

Energy equation: ! Kg X vðruhÞ vðrvhÞ v vT þ þ Yk hk Vk;x  lg r vx vy vx vx k¼1 ! Kg X v vT Yk hk Vk;y  lg þ r ¼ 0: vy vy k¼1

Unless otherwise stated, a highly conductive wall material is considered, and its thermal conductivity is assumed to be 80 W/(m$K). This is because the operation of thermally integrated micro-systems is limited only to highly conductive materials [51]. High wall thermal conductivities make possible efficient heat exchange within the system [19], and a small temperature gradient within the wall can be achieved in the longitudinal direction [45,51]; in contrast, high fuel conversion and power output may be achieved for low wall thermal conductivities, whereas the hot spot formation makes the operation of the system impractical [19,52]. The heat transfer and chemical species boundary conditions at the gas-wall interfaces depend on the location of the interfaces. Two different surfaces are considered: catalytic surfaces as well as the left and right edges of the wall. On the catalytic surfaces, species flux and heat consumption or generation by reactions must be included in the model. The boundary condition for the gaseous species at the specified gas-wall interfaces is written as 


 vðruuÞ vðrvuÞ vp v vu 2 vu vv þ þ  2m  m þ vx vy vx vx vx 3 vx vy
  v vu vv m þ ¼ 0;  vy vy vx

(8)

(7)

Since the heat transfer within the dividing wall can significantly influence the performance of the thermally integrated system [19,20,45,51e53], the energy equation in the solid phase is accounted for:

(12)

The overall heat transfer coefficient, ho, is estimated from heat transfer Nusselt numbers correlations [55]. The effective emissivity, εs∞ , is assumed to be 0.8 [55]. The effective thermal conductivity of the porous catalyst layer is given by leff ¼ εc lg þ ð1  εc Þlm :

(13)

Transport in the porous medium is influenced by both molecular and Knudsen diffusion. The effective diffusivity of species i in the porous medium, Dk;eff , can be expressed as Dk;eff ¼

εm Dk ; tm

1 1 1 ¼ þ : Dk Dk;Knudsen Dk;molecular

(14)

(15)

Here, the Knudsen diffusivity of species k, Dk;Knudsen , is written as

Please cite this article in press as: Chen J, et al., Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.001

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Dk;Knudsen

dpore ¼ 3

sffiffiffiffiffiffiffiffiffiffi 8RT : pWk

(16)

reforming reaction occurring under the conditions studied here [23]. Methanol catalytic combustion can be written as

The average pore diameter, dpore , is defined as dpore ¼

CuO=ZnO=Al2 O3

4Vpore : SBET

(17)

CH3 OH þ 1:5O2 !CO2 þ 2H2 O:

(20)

The overall reaction rate is modeled using the expression determined by Reitz et al. [60]:

Chemical kinetics To design an efficient system and to optimize its operating conditions, it is critical to understand the kinetics of both steam reforming and catalytic combustion [8]. An appropriate model for the kinetics can be critical in accurately predicting the operation of the system [8]. For the temperature range of interest, heterogeneous reactions would be expected to dominate, and initiation of homogeneous reactions may be possible but homogeneous reactions would not contribute significantly, except in the case of methane catalytic combustion as discussed earlier. Detailed homogeneous and heterogeneous reaction mechanisms are employed simultaneously for catalytic combustion of methane. The homogeneous chemistry is modeled with the Leeds methane oxidation mechanism [56,57], which consists of 351 irreversible reactions with 37 species. The heterogeneous chemistry is modeled using the kinetic mechanism developed by Deutschmann et al. [58]. The mechanism consists of 24 elementary reactions involving 9 gaseous species and 11 surface-adsorbed species, including adsorption, desorption, and surface reaction steps. The density of platinum surface sites is taken to be 2.72  109 mol/cm2 [58]. Adsorption of hydrogen and carbon monoxide is assumed to be first-order and second-order, respectively, with regard to empty sites. This mechanism is allowed to interact with the homogeneous reaction mechanism through the radical species such as hydrogen, oxygen, and hydroxyl radicals, as well as the molecules such as hydrogen, oxygen, methane, carbon monoxide, carbon dioxide, and water. More details about this mechanism are available at the website: detchem.com. For methane steam reforming, the homogeneous chemistry is negligible as discussed earlier, and the heterogeneous chemistry is modeled using the mechanism proposed by Karakaya et al. [24]. The density of rhodium surface sites is also taken to be 2.72  109 mol/cm2 [24]. The catalyst/geometric surface area, which is used to describe the dependence of the overall reaction rate on catalyst loading [28], is assumed to be 30 [59]. Despite numerous efforts, there are no detailed reaction mechanisms available for either steam reforming or catalytic combustion of the methanol fuel. Methanol steam reforming can be written as

0:18 0:18   115000 pmethanol poxygen : s_methanol mol gcat $min ¼ 6:0  108 e RT 0:14 pwater

(21)

The catalyst loading is directly related to the rate of the reforming reactions, which in turn affects the rate of heat consumption or generation. A direct comparison of reactor performance between the two systems requires comparable catalyst loadings. Consequently, the same catalyst/geometric surface area 30 is used for both combustion and reforming catalysts. High catalyst loadings are employed to improve the performance of the system. Thermodynamic data are obtained from the provided schemes. The CHEMKIN transport database [50] is used to determine the transport properties for the model. The homogeneous and heterogeneous reaction rates are handled through the CHEMKIN [61] and SurfaceCHEMKIN [62] interfaces, respectively.

Computation scheme

The overall reaction rate is modeled using the expression determined by Cao et al. [22]:

For the base case, an orthogonal staggered grid consisting of 200 transverse nodes by 200 axial nodes is employed. A grid in excess of 80,000 nodes is utilized for the largest dimension. The grid independency of the solution is verified by varying level of refinement. At each of the gas-wall interfaces, the noslip boundary condition is applied, and radiative properties and detailed reaction mechanisms are specified. The conservation equations are solved implicitly through a twodimensional steady-state segregated solver using an underrelaxation factor control method. The second-order upwind scheme is used to discretize the model, and the “SIMPLE” algorithm is employed to solve the pressure-velocity coupling momentum equation. The solution is deemed to be converged as the residuals of the conservation equations are less than 106. Parallel computation is adopted. The breakthrough limit described in the following sections is defined as 99% conversion of the fuel in the reforming channel [52,63]. Note that the conversion is integrated over the transverse direction of the channel. The breakthrough of reactants is crucial not only from the perspective of efficiency, but also from the perspective of downstream processing such as safety and environmental issues needed in micro-chemical systems. Please refer to the reference [42] for more detail information on breakthrough. Note that the operation line determined by these breakthrough limits is an important design criteria [52,63], and will be discussed in detail in the following sections. On the other hand, for clarity, the fuel mass fractions in the combustion channel are multiplied by ten.

  94800 0:088 s_methanol mmol=kgcat $s ¼ 2:9047  1010 e RT p0:715 methanol pwater :

Numerical validation

Pd=ZnO=Al2 O3

CH3 OH þ H2 O!CO2 þ 3H2 :

(18)

(19)

Note that carbon dioxide and hydrogen are assumed to be the only products produced. This is because there is only a very small amount of carbon monoxide produced by the

The model developed above is validated by comparing the numerical results with the experimental data available in the

Please cite this article in press as: Chen J, et al., Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.001

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literature [64,65]. In order to verify the combustion scheme implemented in the present investigation, the experimental results reported by Dogwiler et al. [64] are utilized. The combustor length and gap size are 250.0 and 7.0 mm, respectively. The catalyst temperature distribution measured by thermocouple serves as the energy boundary condition at the gas-wall interfaces. Fig. 2(a) shows the streamwise centerline OH concentration profiles compared to the experimental results. The OH concentration profiles predicted in the present work is shifted axially to match the measured peak OH locations. There is a sharp rise in the concentration of OH radicals along the centerline, indicating the onset of ignition in the gas phase, which is accurately predicted by the present work. The locations of homogeneous ignition is predicted within 16% in all the cases examined. It is shown that the numerical predictions are in good agreement with the experimental data. In order to verify the steam reforming scheme implemented in the present investigation, the experimental results reported by Zhai et al. [65] are utilized. The channel length and gap size are 30.0 mm and 0.5 mm, respectively. The catalytic performance of steam reforming of methane over rhodium has been investigated, and the details of the experimental case is available in the literature [65]. The methane conversion and the selectivity to carbon dioxide are compared to the experimental data available in the literature in Fig. 2(b). It is also shown that the numerical predictions are in good agreement with the experimental data.

Results and discussion The reactor performance for hydrogen production is evaluated by means of conversion, as well as thermal efficiency, which will be defined later. Thermal management is also discussed preliminarily, as it is a key tool for the design of balance for system energy [26].

Thermal behavior The thermal coupling of exothermic and endothermic reactions plays an important role in reactor performance, which have been highlighted in previous studies [66e68]. In this section, simulations are performed to gain insight into the thermal behavior of the system. Fig. 3 shows the centerline profiles of fuel mass fractions in each of the channels at the breakthrough limit for the base case shown in Table 1. The breakthrough line delimits the complete conversion regime in the reforming channel. For slow reforming stream flows, complete conversion can be achieved with high temperatures. In contrast, for fast reforming stream flows, conversion is incomplete due to low temperatures. The thermal coupling in the reactor is important for efficient operation of the system. If the heat flux generated and consumed is locally or globally unbalanced, the operation of the system swings between two opposite extreme situations. When the rate of heat generation is higher than that of heat consumption, a “hot spot” develops due to the increased temperatures. This is an undesirable situation, as hot spots could affect reactor

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operability and control and damage the catalysts on both sides. In contrast, if the rate of heat consumption is higher than that of heat generation, a “cold spot” is formed due to the decreased temperatures, resulting in incomplete conversions on both sides. Fig. 3 shows that for the methane-based system, the ratio of axial distance on the reforming side to that on the combustion side is 2.4 when 99% conversion is achieved. In contrast, the ratio is 4.4 for the methanol-based system. Therefore, the reactor is better balanced thermally for the methane-based system. In contrast, heat consumed and heat generated are not balanced locally for the methanol-based system. In this case, the design of the methanol-based system requires a good solution to balance the heat consumed and generated, especially if high conversions are desired. These will be discussed in detail in the following sections. On the other hand, it is important to note that a sufficient length of the reactor is necessary in order to avoid loss of stability (e.g., extinction and blowout) in the combustion channel and to avoid serious safe problems, since operation of a microscale combustion system is often limited to a narrow window [42]. Fig. 4 shows contour plots of the fuel and hydrogen mass fractions in the reforming channel at the breakthrough limit for the base case shown in Table 1. The breakthrough limit corresponds to an inlet velocity 4 m/s of the combustible stream, as shown in Fig. 3. The combustion reaction takes place very rapidly in each system, and complete conversion is achieved near the entrance, which can be attributed to the high catalyst/geometric surface area employed for both systems. On the other hand, there are significant differences in reforming reaction between methane and methanol, as shown in Fig. 4. For the methane reforming process, the reaction is fast in the vicinity of catalytic surfaces, and chemical equilibrium is reached. In contrast, steam reforming of methanol is much slower than that of methane. At the outlet, 99% fuel conversion, i.e., the breakthrough limit, is achieved for both reforming processes. When the inlet velocity of the reforming stream is larger than the breakthrough limit, there is not enough energy available from the combustion side to drive the endothermic process. Therefore, the fuel conversion is incomplete on the reforming side. Conversely, when the inlet velocity of the reforming stream is smaller than the breakthrough limit, there is enough energy available from the combustion side to raise the temperature of the reforming gas mixture as well as to drive the endothermic reaction. In this case, however, temperatures are high, which will be discussed later. Fig. 5 shows the axial temperature profiles along the centerline of each channel and along the wall at the breakthrough limit for the base case. While there is a significant change in the axial temperature profile between the two systems, the reactor thermal behavior has certain fundamental characteristics in common. Near the entrance to the reactor, the fluid temperature on the reforming side is slightly higher than that on the combustion side, as a higher feed temperature (400 K as listed in Table 1) is considered for the reforming stream. The dividing wall provides a route for transfer of heat from the post-combustion region to upstream for preheating

Please cite this article in press as: Chen J, et al., Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.001

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Fig. 2 e Panel (a) shows a comparison of streamwise centerline OH concentrations after the homogeneous ignition with experimental and numerical results available in the literature [64]. Panel (b) shows a comparison of the methane conversion and the selectivity to carbon dioxide with experimental results available in the literature [65].

both streams. As a result, there is a sharp temperature rise in each of the channels, until reaching a maximum value. After this point, there is no significant temperature gradient in the transverse direction, indicating that the heat exchange within the system is efficient. This can be attributed to the relatively small channels and the high wall thermal conductivity. While the main part of the heat released during the catalytic combustion process is utilized to drive the reforming reaction, the rest is utilized to heat both streams. A smooth distribution of

temperature is achieved in the axial direction and no hot spots are formed.

Power output There are several factors influencing reactor performance, such as maximum power output and throughput. The power output per unit width of the device is computed by assuming that the hydrogen produced by the reforming reaction can be

Please cite this article in press as: Chen J, et al., Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.001

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Fig. 3 e Fuel mass fraction profiles along the centerline of each channel at the breakthrough limit for the base case. For clarity, the fuel mass fractions in the combustion channel are multiplied by ten. The breakthrough limit is defined as 99% conversion of the fuel in the reforming channel.

completely utilized. Note that the energy content of the carbon monoxide produced is not taken into account, as the system is intended to produce hydrogen for use in fuel cell applications. On the other hand, the power output is arguably one of the most important performance measures of a thermally integrated micro-device [69,70]. Fig. 6 is a comparison of throughput and power output at the breakthrough limits between the two thermally integrated systems for various inlet velocities of the combustible stream. The effect of inlet velocity could be considered through variation of the flow rate. When inlet operating conditions, reactor geometry, and catalyst loading are fixed, variations of inlet velocities result in corresponding variations of flow rates and residence times. In this section, a highly conductive wall material is considered as discussed earlier, and its thermal conductivity is assumed to be 80 W/(m$K). The effect of wall thermal conductivity on the reactor performance will be discussed later. A smooth distribution of temperature in the axial direction can be achieved by the adjustment of the thermal coupling in the system [19,20]. In light of this, the throughput, which is represented by the ratio of the inlet velocity in the reforming channel to that in the combustion channel, is used to express the energy balance within the system. Note that the throughput is dimensionless. Interestingly, the flow rate at the breakthrough limits has little effect on the ratio, as will be discussed later. More importantly, it can be easily determined by means of overall energy balance [52]. Stefanidis et al. [52] have provided an effective method for the approximate evaluation of the ratio at the breakthrough limit. On the basis of the same energy input on the combustion side for the two thermally integrated systems, which is obtained by means of the same inlet velocities of the two combustible streams with

appropriately balanced compositions, the inlet velocity in the methanol reforming channel can be increased by more than four (or, more accurately, 4.0e4.8) times higher than that in the methane reforming channel in order to achieve the same conversion (left ordinate in Fig. 6). This is because the standard enthalpy of the methane reforming reaction is approximately 4.2 times than that of the methanol reforming reaction. Note that standard conditions are defined as a temperature of 273.15 K and an absolute pressure of exactly 105 Pa, as well as both of the two reforming reactions are endothermic in nature. The ratio of inlet velocities in the reforming and combustion channels along the breakthrough line varies only slightly, i.e., the flow rate at the breakthrough limits has little effect on the ratio. Therefore, a strategy of isoflow rates ratio can be suggested by the adjustment of the two flow rates on opposite sides of the reactor in an almost linear fashion. This strategy has been presented and discussed in detail in the literature [52]. Fig. 6 illustrates that the amount of power generated from hydrogen for a device of 10.0 mm width for the methanolbased system, which is computed based on the lower heating value of the hydrogen produced using a reference temperature of 298 K, is 3.8e4.6 times more than that for the methane-based system (right ordinate in Fig. 6). The range is slightly less than the ratio range of methanol to methane throughput (4.0e4.8 times), since a higher hydrogen yield can be achieved during the water-gas shift reaction for the methane-based system. The results also indicates that a simple operating strategy can be proposed to achieve variable power output: the flow rates can be adjusted along the breakthrough line to achieve the desired power output. In order to more easily compare the performance of different microchannel reactors, the metric of contact time

Please cite this article in press as: Chen J, et al., Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.001

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Fig. 4 e Contour plots of the fuel and hydrogen mass fractions in the reforming channel at the breakthrough limit for the base case. The breakthrough limit corresponds to combustible stream inlet velocity of 4 m/s shown in Fig. 3.

[16] is employed here, and an analysis of the contact time is briefly made. The contact time is computed as the sum of contact times in each element of the catalytic surface up to 99.9% fuel conversion, i.e., the sum of contact times in the effective, reactive length of the reactor [52,63]. However, when the reforming is incomplete, the contact time is the sum of contact times in each element of the entire catalytic surface [52,63]. A comparison of contact time between the two processes at the breakthrough limits indicates that both reforming processes are feasible at millisecond contact times. The contact time ranges from as low as several milliseconds (under the conditions considered herein) to tens of milliseconds (for the case of low catalyst loading).

Effect of wall thermal conductivity The thermal conductivity of the dividing wall is very important since it determines the upstream heat transfer through the wall for preheating both streams. Fig. 7 shows the effect of

wall thermal conductivity on the maximum wall temperature at the breakthrough limits. The breakthrough limits correspond to combustible stream inlet velocities of 2, 4 and 6 m/s, respectively, shown in Fig. 6. For the methane-based system, the wall thermal conductivity has slight influence on the maximum temperature, since the system is better balanced thermally as discussed earlier. Conversely, the wall thermal conductivity plays a considerable role in the operation of the methanol-based system. Systems with higher wall thermal conductivities lead to lower temperatures. The methanolbased system is strongly affected by the overall energy imbalance, as shown in Figs. 3 and 4, which in turn is affected by the wall thermal conductivity. This effect at low catalyst loading becomes more pronounced, as reported in the literature [19]. To avoid insufficient reactant conversion and hot spots, it is necessary to carefully adjust the ratio of catalyst loadings employed on opposite sides of the reactor [19]. Therefore, to design a high efficient methanol-based system, an alternative solution is required to achieve the overall

Please cite this article in press as: Chen J, et al., Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.001

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Fig. 5 e Axial temperature profiles along the wall and the centerline of each channel at the breakthrough limit for the base case.

Fig. 6 e Comparison of throughput and power output between the methane and methanol reforming processes at the breakthrough limits for various inlet velocities of the combustible stream. The throughput is represented by the ratio of inlet velocities in the reforming and combustion channels.

energy balance, especially if high final conversion is desired. These may include optimization of catalyst loading in the reforming channel. Fig. 8 shows the maximum temperature differences within the wall for different wall thermal conductivities at the same breakthrough limits as illustrated previously in Fig. 7. For both systems, the maximum temperature difference within the wall decreases with increasing wall thermal conductivity. Low thermal uniformity is achieved for highly insulating

materials, resulting in larger differences in wall temperature, up to approximately 270 K. This situation is considered to be undesirable, since hot or cold spots may develop. Hot spots may put severe constraints on reactor materials, cause deactivation of the catalyst, and affect the operation of the system; cold spots can yield poor performance [44]. To achieve the energy balance within the system for highly insulating materials, very careful design is necessary and fully understanding the reactor thermal behavior is also required.

Please cite this article in press as: Chen J, et al., Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.001

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Fig. 7 e Effect of wall thermal conductivity on the maximum wall temperature at the breakthrough limits. The breakthrough limits correspond to combustible stream inlet velocities of 2, 4 and 6 m/s, respectively, shown in Fig. 6.

Effect of channel height The channel height can significantly influence reactor performance [29,30], particularly when the system is operated at short contact times [16]. The dependence of thermal coupling on channel height has been addressed by Zanfir and Gavriilidis [19,20,44]. It has been found that reactor performance is

strongly influenced by overall and local thermal balance in the system, which in turn is affected by channel height. The efficiency of heat exchange within the system is favored at low channel height. In this section, the effect of combustion channel height is evaluated for a fixed flow rate. The same change is made for the reforming channel. Note that the main problem with small gaps is the increased pressure drop that is

Fig. 8 e Effect of wall thermal conductivity on the maximum wall temperature difference at the same breakthrough limits as shown previously in Fig. 7. Please cite this article in press as: Chen J, et al., Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.001

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not addressed in the present work, and further investigations are needed. The effect of channel height on the conversion and maximum wall temperature is shown in Fig. 9 for both systems. The combustion conversion of the two fuels is plotted for different channel heights in Fig. 9(a). The inlet flow rate remains constant, and both base case value and its double value are considered, corresponding to the inlet flow velocities 4 and 8 m/s, respectively, of the combustible stream, as shown in Fig. 6. The effect of channel height can be evaluated by € hler number, which relates the using the transverse Damko transverse diffusion time-scale to the reaction time-scale. The transverse diffusion time-scale is defined as tdiffusion ¼

d2 : 4Dfuel

(22)

The chemical reaction time-scale is defined as follows [71]: 0:5Cfuel;in

: treaction ¼  s_fuel ðS=VÞ þ u_ fuel

(23)

Except in the case of methane catalytic combustion, homogeneous reactions are negligible (u_ fuel ¼ 0) as discussed earlier. These time scales are computed at points located 5.0 and 10.0 mm from the entrance for the methane-based and methanol-based systems, respectively. These locations are sufficiently downstream to allow preheating of both streams, but not too far downstream where both combustion and reforming have proceeded to completion. The transverse € hler number can be used to assess the relative imporDamko tance of reaction kinetics and transport, defined as tdiffusion d2 =4Dfuel 

¼ treaction 0:5Cfuel;in s_fuel ðS=VÞ þ u_ fuel 

d2 s_fuel ðS=VÞ þ u_ fuel : ¼ 2Cfuel;in Dfuel

Dat ¼

(24)

To avoid hot spots and extinction issues, operation of a micro-combustion system is often limited to a narrow window [42]. Fig. 9(a) indicates that the combustion channel height has little influence on the outlet conversion on the combustion side, since nearly complete conversion can be achieved in all the cases examined. For the combustion channel height that is more than double its base value, the methane combustion process is determined by gas-to-solid mass transport, i.e., the reaction is strictly limited by mass transfer (Dat > 10). In contrast, within the range of combustion channel height studied here, the catalytic combustion of methanol is strictly limited by mass transfer (Dat > 10), or is limited by both mass transfer and kinetics (1.0 < Dat < 10). While the diffusion effect is significant, the change in combustion channel height does not significantly affect the outlet conversion due to the intrinsic fast combustion kinetics of both fuels. Fig. 9(a) also indicates that the reforming channel height has no effect on the outlet conversion on the combustion side, and complete conversion is achieved in all the cases studied. Overall, the channel height has little effect on the final conversion on the combustion side due to the fast combustion kinetics. The reforming conversion of the two fuels is plotted for different channel heights in Fig. 9 (b). Similarly, the combustion

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channel height has little effect on the outlet conversion of methane on the reforming side (red lines). The methane reforming reaction is in a mixed control regime (0.2 < Dat < 0.8). However, the chemical reaction time is always shorter than the residence time. As a result, the combustion channel height has no noticeable effect on the methane reforming process. As the combustion channel height varies, similar but more pronounced effects can be observed on methanol conversions in the reforming channel (blue lines). Nevertheless, methanol conversion still remains very high, close to the equilibrium value. Within the range of inlet velocity examined, the methanol reforming reaction is in a kinetically-controlled regime. At higher inlet velocities, breakthrough of reactants may occur for the reforming stream due to the decreased residence time. Therefore, channel heights should be properly designed to ensure sufficient conversion. On the other hand, as the reforming channel height varies, nearly complete methane conversion, which is close to the equilibrium value, is achieved on the reforming side in all the cases examined (green lines in Fig. 9(b) in the web version). As the reforming channel height is increased, the reforming reaction of methane becomes to be limited by both mass transfer and kinetics (1.0 < Dat < 10), or to be strictly limited by mass transfer (Dat > 10). Within the range of channel height and inlet velocity studied, the transverse diffusion time is much less than the residence time. For the methane-based system, the equilibrium conversion on the reforming side is achieved near the entrance to the reactor for all the reforming channel heights studied. In contrast, for the methanol-based system, as the reforming channel height increases, the outlet conversion on the reforming side decreases linearly due to the limitation of residence time (dark yellow lines in Fig. 9(b) in the web version). Similar but more pronounced diffusion effects are observed in the methanol reforming process, based on the analysis on the interplay between ki€ hler number netics and transport using the transverse Damko as discussed above. For both high channel heights and inlet flow rates on the reforming side, the reforming reaction of methanol is strictly limited by mass transfer (Dat > 10), or is limited by both mass transfer and kinetics (1.0 < Dat < 10). The residence time is of the same order of magnitude as the maximum transverse diffusion time, and thus breakthrough of reactants may occur for the reforming stream. The maximum wall temperature is plotted for different channel heights in Fig. 9(c). For the methane-based system, the combustion channel height has little or no effect on the maximum wall temperature (red lines). In the case of methane, the better energy balance between heat generation and heat consumption is achieved at the nominal channel height, as shown in Fig. 3. The reactor is also better balanced thermally for the higher combustion channels examined, and thus there is no significant change in maximum wall temperature. On the other hand, heat consumed and heat generated are not balanced locally for the methanol-based system at the nominal channel height, as shown in Fig. 3. As the combustion channel is increased, the maximum wall temperature decreases (blue lines in Fig. 9(c)in the web version) due to the improved energy balance within the system, even though the methanol conversion might fall on both sides (blue lines in Fig. 9(a) and (b)in the web version).

Please cite this article in press as: Chen J, et al., Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.001

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Fig. 9 e Conversion and maximum wall temperature versus normalized combustion and reforming channel height. The nominal combustion and reforming channel heights are docom ¼ doref ¼ 0.8 mm. The inlet flow rate is kept constant at the base case value and its double value, respectively, corresponding to combustible stream inlet velocities of 4 and 8 m/s shown in Fig. 6. In panel (a), the points are indistinguishable when the reforming channel height varies.

On the other hand, for the methane-based system, the reforming channel height has little or no effect on the maximum wall temperature (green lines in Fig. 9(c)in the web version), because the reactor is better balanced thermally. In contrast, there is a serious energy imbalance occurring in the methanol-based system as shown in Fig. 3, especially for lager reforming channels. The unbalanced heat fluxes, coupled with the lower conversion on the methanol reforming side, lead to higher maximum wall temperatures for higher reforming channels (dark yellow lines in Fig. 9(c)), which is the opposite of that obtained for higher combustion channels (blue lines in Fig. 9(c)in the web version). Overall, at constant inlet flow rates, the channel height has little effect on the performance of the methane-based system, whereas it has a considerable effect on the temperature and

conversion in the methanol-based system. Nevertheless, both systems can be operated at millisecond contact times, and very high conversions can be achieved for both endothermic and exothermic reactions.

Thermal efficiency In general, the thermal efficiency is a measure of energy output divided by energy input. Unfortunately, there are a variety of ways to implement this general definition [72]. In this section, two definitions of thermal efficiency are examined, and then a thermodynamic analysis is made. Fig. 10 shows the thermal efficiency of the two processing systems at the breakthrough limits. The thermal efficiency of the system (hereinafter referred to as the system efficiency) is

Please cite this article in press as: Chen J, et al., Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.001

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defined as the ratio of the lower heating value of the hydrogen produced to the lower heating value of the fuels used Q_ H2 $LHVH2 : h¼ _ Q fuel;reforming $LHVfuel;reforming þ Q_ fuel;combustion $LHVfuel;combustion

hreforming ¼ (25)

The denominator refers to the lower heating value of all the fuels consumed [73]. The heating value is defined to be positive for an exothermic reaction. Fig. 10 shows that the thermal efficiency is approximately 60% for the methane-based system (red dashed-line, left ordinate) and higher than 90% for the methanol-based system (blue dashed-line, left ordinate). When the flow rate varies along the breakthrough line, there is little change in the thermal efficiency for each of the systems. The thermal efficiency defined above does not account for the heat of vaporization of liquid fuels. All things considered, the thermal efficiency would decrease slightly. Such confusion does not exist in the present work, as a high feed temperature (400 K as shown in Table 1) is considered on the reforming side for both systems. This thermal efficiency is regarded as maximum for the process, because the heat loss to the surroundings is not considered in this definition and complete conversion is assumed to be achieved on both sides of the reactor. Since thermal management plays an important role in the design of portable fuel processing systems [26,27], two thermal efficiency definitions are examined to understand the role of heat loss and subsequently develop an effective heat recuperation strategy. The thermal efficiency of the reforming process at the breakthrough limits is also shown in Fig. 10 (right ordinate). This thermal efficiency (hereinafter referred to as the process efficiency) is defined as the standard enthalpy of the reforming reaction at constant pressure divided by that of the combustion reaction in the adjacent channel at constant pressure

  Q_ fuel;reforming $Dr Hqm ðTin Þreforming   : Q_ fuel;combustion $Dc Hqm ðTin Þcombustion

15

(26)

Fig. 10 shows that the process efficiency of the methanolbased system (blue solid line, right ordinate) is higher compared to the methane-based system (red solid line, right ordinate). The advantage of methanol reforming process is more apparent at low inlet flow velocities. In contrast, there is little difference in process efficiency between the two systems at high inlet velocities, although much lower temperatures are obtained at the outlet of the methanol-based system. This is attributed to the much higher inlet velocities for the methanol-based system at the breakthrough limits, which can eventually lead to increased excess enthalpy on the reforming side. While at high inlet velocities the process efficiency of the two systems are comparable (right ordinate in Fig. 10), there is a significant difference in system efficiency between them (left ordinate in Fig. 10). To distinguish between the two definitions of efficiency, it is useful to derive an analytical expression of the relation between them. When complete conversion is assumed to be achieved on both sides of the reactor, the process efficiency is approximated as h¼

n$hreforming $LHVH2   : h$LHVfuel;reforming þ Dr Hqm ðTin Þreforming

(27)

Here, n is the stoichiometric coefficient of hydrogen in the reforming reaction. The relation between system efficiency h and process efficiency hreforming is plotted in Fig. 11. The thermal efficiency of the two systems is also compared with that of the ammoniabased system, since ammonia is of great potential to be an alternative fuel for the production of hydrogen [3]. To better understand the differences between the two definitions, a theoretical fuel, denoted as “pseudo-methane,” is defined. The

Fig. 10 e System efficiency and process efficiency versus combustible stream inlet velocity at the breakthrough limits. The breakthrough limits correspond to the fuel breakthrough points shown in Fig. 6 (dashed lines). Please cite this article in press as: Chen J, et al., Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.001

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physicochemical properties of this fuel are the same as those of methane except that the water-gas shift is assumed to be an irreversible reaction, i.e., complete conversion of the carbon monoxide produced from steam reforming into carbon dioxide is possible. Fig. 11 shows that the system efficiency increases with increasing process efficiency. At sufficiently high process efficiency, heat losses have little effect on the system efficiency. The theoretical maximum value of system efficiency is 100% for all fuels except methane. The maximum thermal efficiency for the methane-based system is approximately 72%, as the heating value of carbon monoxide produced is not taken into consideration. This could partly explain the obvious differences in system efficiency between methane and methanol shown in Fig. 10 (left ordinate). Furthermore, methane is more sensitive to the process efficiency than methanol. The derivative of system efficiency with respect to process efficiency, dh/dhreforming, is studied. The results indicate that as the process efficiency decreases until it reaches a critical value of 20%, the thermal efficiency of the methanebased system decreases faster compared to the methanolbased system. The standard enthalpy of pseudo-methane reforming reaction is 165 kJ/mol, whereas that of methane reforming reaction is 206 kJ/mol. This implies the thermal integration between steam reforming and water-gas shift. In this case, the heat produced by the exothermic water-gas shift reaction is absorbed somehow by the endothermic steam reforming reaction. In the absence of external heat losses, the theoretical maximum value of the pseudo-methane-based system efficiency is 100%, as shown in Fig. 11. In most cases, however, this system is obviously less robust against external heat losses than the methanol-based system. It will be challenging to achieve the same process efficiency between the two

systems under similar conditions, since the operating temperature is entirely different from each other. To achieve higher thermal efficiency, thermal integration should be carried out by recuperating energy from the products to the incoming reactants. Overall, higher system efficiency can be achieved for the methanol-based system due to: higher process efficiency (right ordinate in Fig. 10); the lower sensitivity to process efficiency (Fig. 11); and a low or negligible amount of carbon monoxide produced [74,75]. The process efficiency of the methanol-based system can be improved by decreasing the excess steam in the feed [76], the reforming stream flow rate under non-equilibrium conditions as shown in Fig. 10 (blue line, right ordinate), the external heat losses, and the excess enthalpy of the products. If an effective strategy of heat recuperation is developed, even a small increase in process efficiency can significantly improve the system efficiency, when operating at the low efficiency branch, as shown in Fig. 11. In this case, the system efficiency is particularly sensitive to the process efficiency. Due to significant heat losses, the process efficiency of a practical methanol-based system is often in the low efficiency regime, in which the system efficiency increases linearly with increasing process efficiency, as shown in Fig. 11. Consequently, it is highly desirable to develop an effective heat recuperation strategy. Fortunately, microreactor units can be “numbered-up” or “scaled-out” to form larger stacks [77]. In this case, heat losses can be reduced, and adiabatic operation would be achieved. Consequently, process efficiency that is equal to or higher than 40% is feasible. Conversely, when the system is operated in the moderate and high efficiency regime, as shown in Fig. 11, a strategy of heat recuperation might not be very effective, since the system efficiency has been found to be less sensitive to the process efficiency.

Fig. 11 e System efficiency versus process efficiency under the conditions of complete fuel conversion on both sides of the reactor. Please cite this article in press as: Chen J, et al., Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.001

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Conclusions The design and operation of thermally integrated microreactors for the production of hydrogen were studied numerically. A numerical model was developed to evaluate the role of different variables affecting the operation of the system. Two definitions of thermal efficiency were examined and a thermodynamic analysis was made. The main points can be summarized as follows:  Both methanol- and methane-based thermally integrated systems can operate on a millisecond time-scale, provided that operating parameters are properly designed.  Methanol-based thermally integrated systems can be more energy-efficient. They have the advantage of simplifying the system design, effectively improving power output, and significantly increasing energy efficiency at the overall system level.  Variable power output is feasible by simply adjusting the flow rates. The ratio of the flow rates on opposite sides of the reactor is an important design parameter and must be carefully adjusted to improve efficiency and eliminate hot spots.  The channel height has little effect on the reactor performance, especially for the methane reforming process, when the flow rate is kept constant.  The wall thermal conductivity is essential to the design and optimization of the system. Highly conductive materials make possible efficient energy balance within the system, whereas highly insulating materials limit the transfer of heat within the wall, resulting in hot spots. Since the physical phenomena changes in the third dimension could be significant, three dimensional models are necessary to more accurately predict the reactor performance. This shows promise but is a difficult task if detailed chemistry is used. Higher pressure drops should also be considered in choosing a practical reactor dimension. Several additional topics also need to be further investigated. The interesting topics include the effect of catalyst loading, optimization of flow field, and optimization of catalyst activity distribution.

Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 51506048) and the Fundamental Research Funds for the Universities of Henan Province (No. NSFRF140119).

references

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