Reformer

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Ind. Eng. Chem. Res. 2005, 44, 4982-4992

CFD Simulations of Coupled, Countercurrent Combustor/Reformer Microdevices for Hydrogen Production S. R. Deshmukh and D. G. Vlachos* Department of Chemical Engineering and Center for Catalytic Science and Technology (CCST), University of Delaware, Newark, Delaware 19716-3110

Two-dimensional computational fluid dynamics (CFD) simulations are used to study spatially segregated, multifunctional, microchemical devices for hydrogen production. In particular, coupling between homogeneous propane combustion and catalytic ammonia decomposition on a Ru catalyst is studied in a microdevice consisting of alternating combustion and decomposition channels as a function of flow rate and materials conductivity in the countercurrent flow configuration. It is found that the high temperatures generated via homogeneous combustion lead to high conversions in short contact times and thus to compact devices. Different performance measures are evaluated to assess the operability of the device. Sufficiently high ammonia flow rates serve a dual purpose by lowering device temperatures and enabling the production of larger flow rates of hydrogen. Finally, it is shown that device operation is limited only to highconductivity materials and fast ammonia flows. Introduction Microchemical devices are actively being researched as next generation portable power sources due in part to the superior gravimetric energy density of typical liquid fuels compared to Li-based batteries.1 Such a power generation device may consist of a fuel processor, which acts as a hydrogen source, coupled with a proton exchange membrane (PEM) fuel cell. Hydrogen production can be achieved through a number of reactions,2 viz. steam reforming, partial oxidation, and autothermal reforming of hydrocarbons or alcohols, or cracking of ammonia (for simplicity, we refer to all these routes as “reforming”). Most of these reactions have been carried out in microdevices.3-10 The ammonia decomposition route is particularly attractive for PEM fuel cells,11,12 as its products are free of CO that poisons the fuel cell catalyst and, thus, this route could minimize downstream processing, by eliminating the water-gas shift and the preferential oxidation of CO steps. Among the aforementioned hydrogen production routes, most reactions are endothermic. Therefore, energy must be supplied via an exothermic reaction, such as combustion. The thermal coupling between a reformer and a combustor becomes then an important part of developing stand-alone, multifunctional microdevices where heat exchange and chemical reaction are coupled in the same device. Thermal coupling between a combustor and a reformer can be achieved in a number of ways. One strategy is to use “direct” coupling where both reactions are carried out simultaneously in the same reactor using a suitable bifunctional catalyst. This idea has typically been limited to “one-fuel” systems on account of the complexity of the process.13-17 A second strategy is “temporal” coupling of the two reactions, where the exothermic and endothermic reactions are alternately carried out in the same reaction chamber. The reverse flow reactor is a possible avenue for achieving this and has been well-investigated, among others by Dudukovic * To whom correspondence should be addressed. Tel.: (302) 831-2380. Fax: (302) 831-1048. E-mail: [email protected].

and co-workers, for example.18-28 This approach has been successful for weakly exothermic streams where the energy is “trapped” within the reactor. However, the valves necessary for periodic reversal are susceptible to failure. Also, its extension to highly exothermic combustible mixtures or carrying out periodic operation at the microscale has yet to be demonstrated. The final heat integration strategy involves “spatial segregation”. In this approach, the exothermic and endothermic reactions are carried out in different chambers that are separated by a heat-conducting medium (a multifunctional device). Spatial segregation allows an independent choice of fuel, of catalysts, and of reaction conditions for the combustor and the reformer. This flexibility in conjunction with the small feature size of a microdevice, which facilitates heat transfer, renders this approach suitable for microdevices. It is this approach that is addressed in this paper. A summary of investigations on spatially segregated multifunctional reactors is presented in Table 1. Several issues are worth pointing out. Energy has usually been generated via catalytic combustion. Most previous work has focused on the meso- and/or the macroscale where reactants are often preheated. Additionally, heat- and mass-transfer correlations have often been employed in one-dimensional (1D) models to capture the relevant physics. However, the 1D assumption breaks down in gas-phase microburners.29,30 Homogeneous combustion carried out in the gas phase offers certain advantages over catalytic combustion, such as elimination of catalyst along with its associated problems of deactivation and regeneration. Furthermore, higher temperatures can easily be achieved via homogeneous combustion, if appropriately managed to avoid mechanical failure, to drive equilibrium-limited, endothermic reactions to complete conversion at very short time scales. The latter issue is key to compactness whereas the former eliminates or minimizes downstream processing including separations and additional reactors. In this paper, we show that self-sustained homogeneous propane combustion coupled with ammonia de-

10.1021/ie0490987 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/28/2005

Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005 4983 Table 1. Summary of Literature Investigations on Spatially Coupled Reactorsa reaction system

reactor dimensions

type of study (model/experiments)

ref

CH4 steam reforming (WGS and reverse methanation) on Ni-Mg-Al2O3 coupled with CH4 oxidation (homogeneous and catalytic)

L ) 50 cm

1D steady-state dispersion model

39

CH4 reforming coupled with CH4 oxidation (using proprietary catalysts)

L ) 25 cm

experiments Tin ) 653 K (reformer); Tin ) 803 K (combustor)

40

C2H6 dehydrogenation on Pd coupled with CH4 combustion on Pd

L)1m W ) 2 mm T ) 2 mm H)1m

2D steady-state dispersion model Tin ) 923 K

41

CH4 reforming coupled with CH4 combustion (Ni/Ni-Cr foam on Al2O3 as catalyst for both reactions)

L ) 15 cm D ) 18 mm T ) 2 mm

experiments (tubular reactor) Tin ) 373-573 K

42

CH4 steam reforming (WGS and reverse methanation) on Ni-MgO-Al2O3 coupled with CH4 oxidation on Pt-Al2O3

L ) 40 cm (bench scale) L ) 12 m (industrial)

experiments and 1D steady-state model Tin ) 650-800 K

43

CH4 steam reforming coupled with CH4 oxidation (homogeneous and catalytic)

L)1m

1D steady-state model

44

C2H6 dehydrogenation (homogeneous) coupled with CH4 combustion on Pt-Al2O3

L ) 30 cm D ) 4 mm

experiments (tubular reactor) and 1D PFR model

45

CH4 steam reforming (WGS and reverse methanation) on Ni-Mg-Al2O3 coupled with CH4 oxidation on Pt

L ) 30 cm W ) 1-4 mm T ) 0.5 mm

2D steady-state model; Tin ) 793 K

46

CH4 steam reforming on Rh (with Pt-ceria extended reactor for WGS) coupled with CH4 oxidation on Pt

L ) 8 cm W ) 4 mm T ) 0.1 mm H ) 5 cm

experiments and CFD simulations with no wall Tin ) 573 K (reformer); Tin ) 298 K (combustor)

47

CH4 steam reforming on Rh/Al2O3 coupled with CH4 or H2 combustion on Pd/Al2O3

L ) 10 cm W ) 3 mm H ) 2 cm

experiments and 1D transient model Tin ) 673-873 K

48

CH3OH steam reforming on Cu/ZnO coupled with CH3OH combustion on cobalt oxide

L ) 3 cm W ) 0.32 mm T ) 0.2 mm

experiments

49

CH4 steam reforming (WGS and reverse methanation) on Ni-Mg-Al2O3 coupled with CH4 oxidation on Pt

L ) 30 cm W ) 2 mm T ) 0.5 mm

2D steady-state model; Tin ) 793 K

50

C4H10 combustion on Pt-Al2O3 coupled with NH3 cracking on Ir-Al2O3

L ) 3 mm W ) 200 µm H ) 500 µm T ) 2 µm

experiments in a suspended tube reactor and heat-transfer modeling

10

CH4 (hexane and isooctane) steam reforming on a Ni-GIAP-3 catalyst coupled with H2 oxidation on Pt-Al2O3

L ) 20 cm T ) 1 mm W ) 1.5 cm H ) 7 cm

experiments and 2D mathematical modeling Tin ) 293 K (combustor) Tin ) 473 K (reformer)

51

a WGS stands for the water-gas shift reaction. L is the length, W is the width of the reactor channel, D is the diameter of the inner tube in a tubular reactor setup, T is the wall thickness, and H is the height (3rd dimension).

composition (catalyzed by Ru) in the countercurrent flow configuration is possible in a spatially coupled, multifunctional reactor with alternating combustion and reforming channels, as depicted in Figure 1. To the best of our knowledge, these are the first 2D computational fluid dynamics (CFD) studies of multifunctional microscale reactors. The operation characteristics of these coupled reactors are analyzed with special emphasis on the effects of wall thermal conductivity and operating conditions. Finally, operation maps, which provide the “useable” parameter space of these devices, are presented. Chemistry Models A premixed, stoichiometric propane/air mixture is fed to the inlet of the combustion channel. The homoge-

neous propane combustion is modeled as an irreversible, one-step reaction

C3H8 + 5O2 f 3CO2 + 4H2O

(1)

with the rate expression proposed by Westbrook and Dryer31

[ ]

kmol ) 4.836 × m3s 1.256 × 108 J/kmol CC3H80.1CO21.65 (2) 109 exp RT

σ C 3H 8

(

)

where concentrations are in kmol/m3. Pure ammonia flows countercurrently into the reforming channel, which has Ru catalyst deposited on

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Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005 Table 2. Rate Constants for Ammonia Decomposition Reaction Taken from Ref 32a

Figure 1. Schematic of a multifunctional microreformer/microcombustor device. The device length is 1 cm. The combustion channel is 600 µm wide, the reforming channel is 300 mm wide, and the wall is 300 mm thick. Due to symmetry, only half of each channel is simulated. The half channels along with the wall are shown in the schematic. The “centerlines” are the axes of symmetry in each channel. The blowups are an example of the adaptive mesh used. A finer mesh in the reaction zone and a coarser mesh in the postreaction zone are used.

the channel walls. The ammonia decomposition reaction occurs at the catalytic channel wall resulting in hydrogen production

2NH3 f N2 + 3H2

(3)

and is modeled based on our previous work. A microkinetic model of elementary-like reaction steps was proposed, and the detailed mechanism was validated against microreactor experimental data.32 Finally, a computer-aided model reduction methodology was used to obtain the following reduced rate expression

σNH3 )

(

- 2(k4ω2 - k3PN2)

k11 1+ P + k12 NH3

x

k1 P +ω k2 H2

)

2

(4)

where

ω)

x

k3 P + k4 N2

x

k2 k7k9k11 P P -0.5 k1 2k4k10k12 NH3 H2

(5)

This rate expression captures fairly accurately the conversion of ammonia over a wide range of temperatures and eliminates the need of using multicomponent reacting flow simulations without sacrificing on accuracy. The relevant reaction rate constants are given in Table 2. Governing Equations and Numerics The simulated microreactor is a parallel plate reactor with alternating combustion and reforming channels separated by walls, as shown schematically in Figure 1. The device is 1 cm long. The combustion channel is 600 µm wide, the reforming channel is 300 µm wide, and the wall separating the two channels is 300 µm thick. The height of the channel is typically of the order of 5-10 mm in the experiments of ref 11. As a result, a 2D representation of the system is reasonable because of the large (height to width) aspect ratio. The dimensions chosen here have been suggested by previous CFD simulations and/or experiments on uncoupled compo-

reaction no.

sticking coefficient (unitless) or pre-exponential factor (s-1), A

activation energy (kcal/mol), E

1 2 3 4 7 9 10 11 12

1 1.0 × 1013 1 1.0 × 1013 1.0 × 1011 1.0 × 1011 1.0 × 1011 1 1.0 × 1013

1.9 23.7 14.1 37.2 19.1 17.5 13.2 0 18.2

a k ) A exp(-E /RT), where A is the pre-exponential factor i i i i and Ei is the activation energy of the ith reaction or ki ) (Ai/Γm)xRT/2πM exp(-Ei/RT) for adsorption steps i ) 1, 3, and 11, where Ai is the sticking coefficient, Γ is the total site density (e.g., 1015 sites/cm2), M is the molecular weight of the gas-phase species, m is the number of surface species (including vacancies) on the reactant side, R is the universal gas constant, and T is the temperature in K. While the activation energies in the full model are coverage dependent, the activation energies listed here are for θN* ) 0.3 (see ref 32 for rational of choosing this coverage value).

nents.11,29,30,32,33 The alternating channel configuration allows solving only half of each channel, due to symmetry, plus the wall connecting them. At these length scales, the continuum approximation is still valid. However, in contrast to large scale devices, radical quenching becomes important34 unless the materials are properly prepared.35,36 The commercial CFD software Fluent (version 6.1.22)37 is used to solve the 2D steady-state continuity, momentum, energy, and species conservation equations in the fluid (gas) phase and the heat equation in the solid phase using a finite volume approach. An adaptive meshing scheme is used for the discretization of the differential equations. The computational mesh is initialized with 100 axial nodes, 50 radial nodes for the combustion channel and the wall sections, and 25 radial nodes for the reforming channel. This discretization translates into a total of about 13000 nodes. This initial mesh is adapted and refined during a calculation to increase the accuracy of the solution in regions of high gradients. Specifically, additional nodes are introduced to refine the mesh using the tools built in the computational software so that the normalized gradients in temperature and species between adjacent cells are lower than 10-6. Adaptation is performed if the solution has not converged after about 106 iterations or when the residuals are around 10-6. This last threshold, while not optimized, is meant to strike a balance between cost and probability for convergence. Specifically, mesh refinement before achieving complete convergence reduces the computational effort, but a too early refinement, i.e., in a few iterations, may lead to refinement in wrong regions. After mesh refinement, a total of 25000-200000 nodes are used. Such an adaptive meshing strategy, starting with a relatively coarse initial mesh followed by refinement in regions of large gradients, achieves an adequate balance between accuracy and computational effort. The fluid density is calculated using the ideal gas law. The individual properties of various gaseous species, such as thermal conductivity, are calculated using the kinetic theory of gases, whereas the specific heats are determined as a function of temperature using polynomial fits from the thermodynamic database available

Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005 4985

in Fluent. Mixture properties, such as specific heat and thermal conductivity, are calculated from pure component values based on the mass-fraction weighted mixing law. Binary species diffusivities are determined using the Chapman-Enskog equation and then are used to calculate the multicomponent mixture diffusivities. For the solid wall, a constant specific heat and an isotropic thermal conductivity are specified. Given that material conductivity varies with temperature and more importantly with the material chosen, simulations are carried out over a wide range of conductivities. The model boundary conditions are described next. Danckwerts boundary conditions are implemented for the species and temperatures at the inlets to better mimic experimental conditions. Both gases enter the channels at room temperature (300 K) with a uniform, flat flow velocity. The reactor exits are held at a fixed pressure of 1 atm and the normal gradients of species and temperature, with respect to the direction of the flow, are set to zero. Symmetry boundary condition is applied at the centerline of both channels, implying a zero normal velocity and zero normal gradients of all variables. No-slip boundary condition is applied at each wall-fluid interface. Overall, the device is adiabatic; i.e., no heat losses occur through the side walls. Based on previous work, radiation losses were found to play a secondary effect on the operation of microburners29 and hence have been neglected. Continuity in temperature and heat flux is applied at the fluid-solid interfaces. It should be noted that neither heat- nor mass-transfer correlations are employed since detailed transport within the solid and fluid phases is explicitly accounted for. The full problem is solved via a segregated solver using an under-relaxation method. Convergence of the solution is monitored through the residuals of the governing equations and the L2 norm of successive iterations of the solution. The solution is deemed converged when the residuals of the equations as well as the L2 norm of successive iterations are less than 10-9. The coupling of the heat equation in the wall and the reacting flow equations makes the problem stiff due to the disparity in thermal conductivity between the gases and the wall. Simulations were performed on a 60 processor Beowolf cluster, each processor being a 2.4 GHz Pentium Xeon with 2 GB RAM. Parallel processing using a MPI (message passing interface) was used to speed up the most demanding calculations. In most cases, multiple simulations were run simultaneously on the Beowolf cluster, i.e., one job per node. Typical simulation times varied from about 8 h (for lower flow rates and high wall thermal conductivity) to a few days (for higher flow rates and low wall thermal conductivity) on a single processor depending on the stiffness of the problem. Natural parameter continuation was employed to study the effect of various operating parameters, the results of which are presented below. Microdevice Characteristics for Low Ammonia Flow Rates To obtain an ignited stable solution, initial calculations are performed with a highly conductive wall to achieve better heat recirculation and a low ammonia flow rate to minimize heat removal from the reforming channel. The propane/air inlet flow velocity is 0.5 m/s (this corresponds to the flame location being closest to the inlet, i.e., a very stable situation30), whereas the

Figure 2. Contours of (a) temperature (K), (b) conversion in both channels, (c) propane combustion rate (kmol/m3/s), and (d) velocity (m/s). The C3H8/air and NH3 streams flow in a countercurrent configuration with an inlet flow velocity is 0.5 and 0.05 m/s respectively. The wall thermal conductivity is 200 W/m/K. The solid arrows indicate the flow direction of each stream. The entire structure with all walls is shown only in panel a.

ammonia inlet flow velocity is 0.05 m/s. The corresponding residence times are 20 ms for the propane/air channel and 200 ms for the ammonia channel. Flow velocities here and below are based on an inlet temperature of 300 K. The wall is assumed to be made of a highly conductive material with a thermal conductivity of 200 W/m/K. Typical contours of temperature, conversion, combustion rate, and velocity for countercurrent flow are shown in Figure 2. Figure 3 shows the corresponding temperature and conversion profiles along the centerline of each channel, i.e., the bulk, gas-phase values along the axis of symmetry of a channel, as well as the reaction rate profiles for propane combustion in the bulk and near the wall and for ammonia decomposition at the surface of the wall. Our results demonstrate self-sustained operation within the microdevice with nearly complete conversion of ammonia and propane. Figures 2 and 3 indicate that the high conductivity of the material results in a nearly isothermal wall. The role of the wall as a heat recirculating medium in these integrated microdevices is similar to that previously observed in a single microburner.29,30 Homogeneous combustion generates energy that is transferred relatively fast to the wall. The fast conducting wall enables backward heat transfer toward the entrance of the propane/air mixture to cause ignition of the combustible mixture; i.e., by recirculating energy, the wall serves as an ignition source. Similarly, the ammonia stream is heated by the hot wall and reaches the wall temperature near its entrance, a feature attributed to the fast heat transfer between the gas and the surface for such a narrow channel. This heat transfer from the ammonia stream to the combustion stream renders the stability of the coupled device higher than that of a microburner alone. The spike in the temperature profile along the centerline of the combustion channel marks the homoge-

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Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005

Figure 4. Transverse profiles of (a) temperature (K) and (b) conversion at various distances from the combustion (C3H8/air) channel entrance. The parameters are the same as in Figure 2.

Figure 3. Profiles of (a) temperature (K), (b) conversion, and (c) reaction rates at the centerline of the combustion and reforming channels, as a function of distance from the combustion (C3H8/ air) channel entrance. The shaded regions in panel (a) indicate the direction of heat exchange. The wall preheats the entering gases. The high propane reaction rate marks the combustion zone, associated with the temperature peak, within which almost complete propane conversion is achieved. The parameters are the same as in Figure 2.

neous combustion zone. The maximum temperature reached in this case is less than the adiabatic flame temperature of propane (∼2380 K) and also the melting point of the Ru catalyst (∼2523 K). Most of the propane

is oxidized within this zone, as seen from the corresponding conversion profiles in Figure 3b. The reaction rate of propane combustion is actually higher near the wall due to preheating of the gases by the wall; i.e., the reaction first starts near the wall (see the localized reaction zones near the wall in Figures 3c, 2c, and 6b). The heat generated there is transferred transversely to the fluid, which is warmed to the ignition temperature to form a stable flame along the centerline. Ammonia decomposition starts at the opposite side and quickly reaches chemical equilibrium at these high temperatures. Figure 4a illustrates transverse temperature gradients in the microdevice at various locations of the device measured from the propane entrance. Significant transverse gradients are observed near the inlets that are larger in the homogeneous combustion channel than the catalytic ammonia decomposition channel. Large gradients in the transverse direction of the flow are


Figure 5. Contours of (a) temperature (K) and (b) propane combustion rate (kmol/m3/s) for an NH3 inlet flow velocity of 0.2 m/s. Panels (c) and (d) indicate the corresponding contours for an NH3 inlet flow velocity of 0.4 m/s. With increasing ammonia flow rate, the device temperature decreases and the flame location moves downstream of the combustible entrance. The other parameters are those of Figure 2. The solid arrows indicate the flow direction of each stream.

consistent with our previous work on microburners29,30 and reflect the fact that heat generation is faster than heat removal by transverse conduction despite operating at the microscale. This aspect also rationalizes the need for 2D simulations of gaseous microburners mentioned in the Introduction. Past the reaction zones of both channels, the temperature gradients decrease rapidly and within ∼10% of the length into the channel, i.e., ∼1 mm; the transverse temperature profile is fairly uniform. Similar behavior is also observed for mass transfer, as seen from the conversion profiles in Figure 4b. From a practical standpoint, the wall temperatures are so high for these conditions that it would be impossible to operate such a device for a long period of time. However, it is entirely possible to attain lower wall temperatures, especially at higher flow rates of ammonia where more power is removed from the combustible mixture. The effects of ammonia flow rate and wall thermal conductivity are discussed next in order to develop suitable strategies of operation. Effect of Ammonia Flow Rate In an integrated device, an objective is to recover power from the propane/air combustion to produce the maximum flow rate of hydrogen via ammonia decomposition. When the ammonia flow rate is too high though, extinction of the combustion chemistry can happen. The above chosen flow rate of ammonia was low in order to get self-sustained operation. Once the system is on an ignited branch, the flow rate of ammonia can gradually be increased to produce higher flow rates of hydrogen until extinction occurs. This calculation is enabled by natural parameter continuation, where each ignited solution is attained starting from the previously computed, converged solution.

Figure 6. Profiles of (a) temperature (K) and (b) propane combustion rate (kmol/m3/s) as a function of distance from the C3H8/air entrance for various NH3 inlet flow velocities. The other parameters are those of Figure 2.

Figure 5 shows the contours of temperature and propane combustion rate for two higher ammonia flow velocities at a high wall thermal conductivity of 200 W/m/K. The corresponding longitudinal profiles are depicted in Figure 6. For low ammonia flow velocities, little power is removed from the combustion channel, and the resulting device temperatures are high (compare Figures 2a, 3a, 5a, 5c, and 6a). On account of these high temperatures, the propane combustion rate near the wall is highest, as shown in Figure 6b. High wall temperatures result in the fluid temperature reaching the ignition value early in the channel and the flame being stabilized close to the propane/air inlet. For higher ammonia flow velocities, more power is removed from the combustion channel, and the resulting temperatures are lower as shown in Figure 6a. In turn, these lower temperatures result in lower reaction rates near the leading edge of the wall, which translates to the combustible fluid reaching the ignition temperature further downstream. Thus, increasing the ammonia flow rate causes the propane/air flame to be stabilized further downstream and the combustion to be more localized near the channel center. This observation is consistent with the picture emerging when one increases the outside heat-transfer coefficient of a single combus-

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Figure 7. Profiles of centerline temperatures (a) and the maximum temperature (b) for an ammonia flow velocity of 0.05 m/s and various wall thermal conductivities. Panels (c) and (d) show the same quantities for an ammonia inlet flow velocity of 0.35 m/s. Points in panels (b) and (d) are simulation data and the lines are just a guide to the eye. Lower wall thermal conductivities cause larger temperature gradients. Higher ammonia flow rates reduce the average operating temperature of the device but also enhance the temperature gradients. The other parameters are those of Figure 2.

tion channel to account for possible heat transfer/loss from a combustion channel.29 At high enough flow rates of ammonia, the power generated is insufficient to keep up with that needed for hydrogen production and stability is lost. In previous work, two types of flame stability loss were found, namely, blowout for higher flow rates of a combustible mixture and extinction for low combustible flow rates.29 The maximum allowable ammonia flow velocity that permits sustained combustion is hereafter termed critical velocity. At the critical velocity (∼0.425 m/s or a residence time of τd ) 23.5 ms for these conditions), a stabilized flame is found up to a maximum distance of ∼20% into the channel from the propane/air entrance; i.e., no blowout type stability loss is observed for the combustion channel for this propane/air flow velocity. A final observation for higher ammonia flow velocities is that larger transverse gradients in temperature develop in the gas phase as shown in Figure 6a. These larger gradients probably arise from the more localized nature of combustion. The wall temperatures that hold the key for the thermal stability of the device are, however, uniform; i.e., small gradients are seen within the wall. An important outcome is that at higher

ammonia flow rates the wall temperatures are reasonably high to rapidly drive ammonia decomposition (an endothermic reaction) to equilibrium but within the range of high-temperature materials and catalysts. Thus, high ammonia flow rates serve a dual purpose of lowering the device temperatures to meet the materials stability limit while simultaneously producing higher hydrogen flow rate for use in fuel cells. Effect of Wall Thermal Conductivity In our previous work on microcombustion, we have found that the materials of construction play a dual role.29,30 In particular, very conductive materials transfer heat upstream, enabling ignition but give rise to significant heat losses. At the other extreme, insulating materials retain the energy within the system but do not transfer heat upstream, leading eventually to loss of flame stability by lacking ignition. As a result of the competition between longitudinal and transverse heat transfer, an optimum range of materials exists in terms of conductivity that allows maximum power transfer without stability loss. Aside from the issue of reactor stability, low conductivity materials result in hot spots,


where the wall temperature is locally very high, and in significant longitudinal temperature gradients, which are undesirable from a materials mechanical stability point of view. While these earlier studies underscored the substantial role of walls in heat recirculation and stability, they employed a uniform external heattransfer coefficient via Newton’s law of cooling. They are thus more appropriate for a stand-alone microburner immersed in an stagnant fluid. The above results indicate that substantial temperature gradients may exist in a microdevice, and while an endothermic reaction may remove power near its entrance, the same reaction can become a heat supplier near the combustible mixture inlet. It is therefore important to exploit the effect of material of construction of multifunctional microdevices. The wall thermal conductivity is found to have a significant effect on the operation of an integrated microchemical device. Simulations were performed for various wall thermal conductivities corresponding to highly conductive materials, such as metals, medium conductivity ceramics, and insulators. Figure 7a shows the temperature profiles along the device for three different wall thermal conductivities at a propane/air flow velocity of 0.5 m/s and an ammonia inlet flow velocity of 0.05 m/s. As the thermal conductivity is decreased, temperature gradients develop within the wall that may be undesirable for mechanical stability. The exothermic combustion reaction and the endothermic reaction occur at opposite ends of the reactor, and the wall serves as the primary means of thermal coupling between the two reaction zones. For high wall thermal conductivity, the coupling between the two reaction zones is strong and small longitudinal temperature gradients are seen in the walls of the microdevice. Furthermore, wall temperatures are lower for very conductive materials, yet too high for these low flow rates of ammonia. Figure 7b shows the maximum temperatures in each of the channels as a function of wall thermal conductivity. Consistent with the discussion above, higher maxima in temperature are seen for lower thermal conductivities. In fact, heat recirculation coupled with the local trapping of the heat due to low thermal conductivity causes the maximum flame temperature to exceed the adiabatic flame temperature for the most insulating materials examined here. For higher ammonia flow rates, the wall thermal conductivity effects are more pronounced. Figure 7c shows the temperature profiles along the device for an ammonia inlet flow velocity of 0.35 m/s and Figure 7d shows the corresponding maximum temperatures. The removal of higher power with increasing ammonia flow rate causes reduction in device temperatures (compare Figures 7a and 7c). Furthermore, the axial gradients in the device are enhanced with increasing ammonia flow rate. For example, for a low thermal conductivity of 2 W/m/K, the axial gradients increase from about ∼200 K (for an ammonia velocity of 0.05 m/s) to as high as ∼1000 K (for an ammonia velocity of 0.35 m/s), whereas the average device temperature is lowered from ∼2300 K to ∼1550 K. The transverse temperature gradients between the combustion and reforming channels also rise with increasing ammonia flow rates. The gain resulting from the lowering of the working temperatures by increasing the ammonia flow rate is

Figure 8. Maximum wall temperatures as a function of (a) ammonia flow velocity and (b) wall thermal conductivity. The lines connect the simulation points. At low ammonia inlet flow velocities (e.g., 0.10 m/s), the device temperatures are too high, rendering common fabrication materials and catalysts unusable. Use of higher ammonia flow velocities (e.g., 0.35 m/s) and high thermal conductivities could provide robust operation. The shaded region illustrates the materials stability limit imposed from a maximum allowable wall temperature of 1500 K.

counterbalanced by the larger temperature gradients (except for high conductivity materials). Operation Maps for Multifunctional Devices Materials stability in terms of the maximum allowable wall temperature is the first important operability criterion. In the previous sections the effect of ammonia flow velocity and wall thermal conductivity on the temperature distribution in microdevices was investigated. The results from these are summarized in Figures 8a and 8b, where the maximum wall temperature attained is depicted as a function of wall thermal conductivity and ammonia velocity, respectively. The melting points of typical fabrication materials and common catalysts are also depicted in Figure 8a (their thermal conductivity value is reported at 1000 K38). An arbitrary temperature threshold of 1500 K is set as an upper materials stability limit typically employed in short contact time high-temperature reactors. The shaded regions in the figures indicate the operation window created by the interplay of the materials and

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Figure 10. Power generated from the hydrogen produced as a function of wall thermal conductivity for various ammonia flow rates. The lines connect the simulation points. The shaded region depicts the operation window delimited by a maximum allowable wall temperature of 1500 K (lower bound) and a critical ammonia velocity (upper bound).

Figure 9. Conversions of (a) propane and (b) ammonia as a function of ammonia flow velocity for various wall thermal conductivities. The lines connect the simulation points. The shaded region depicts the operation window delimited by a maximum allowable wall temperature of 1500 K (materials stability limit) and the critical ammonia flow velocity.

reactor stability limits. It is clear that the choice of materials and ammonia flow rates is restricted to a narrow window of very conductive materials and fast ammonia flows. The latter must be below the critical extinction value. We believe that this behavior is generic for microchemical devices with combustion and reforming channels coupled in the countercurrent configuration. However, specifics of results depend to a certain extent on the propane/air flow rate and the fuel itself and deserve further study. Another measure of device performance is the conversion obtained in the individual combustion and reforming channels. Figures 9a and 9b show the propane and ammonia conversions, respectively, as a function of ammonia inlet flow velocities for selected wall thermal conductivities. The shaded areas again depict the operation window delimited by the maximum materials temperature and the critical ammonia flow velocity. At relatively low ammonia flow velocities (e.g., up to ∼0.30 m/s), temperatures are high and conversions of both streams are high irrespective of the material conductivity. At higher ammonia flow rates, the high temperatures realized for the lower wall thermal conductivity materials allow higher conversion. However, the formation of hot spots make the operation of the device

impractical (the upper bound of the shaded area in Figure 9a depicts exactly this limit). For highly conductive materials, breakthrough of small fractions of the reactants is observed as the ammonia flow rate approaches the critical value. This issue sets yet another constraint in operation. The maximum power that can potentially be generated from a device is arguably one of the most important performance measures. The power generated is a function of the conversion as well as the ammonia flow velocity. Figure 10 shows the prediction based on 100% fuel cell efficiency as a function of wall thermal conductivity at various ammonia flow velocities. The shaded region denotes again the possible operation window. The upper bound for the power generation is determined by the maximum hydrogen produced before extinction occurs. On the other hand, the lower bound is determined by the materials stability limit because low ammonia flow rates cause unacceptably high device temperatures. Figures 9 and 10 indicate that sufficiently high ammonia flow velocities (e.g., ∼80% of the critical value) are best for device operation by striking a balance between high conversions and high hydrogen production rates without destroying the device. Conclusions In this paper, the operation of coupled microreactors was explored in a parallel plate device in the countercurrent flow configuration, with alternating exothermic and endothermic reaction channels separated by a wall, using 2D CFD simulations. Homogeneous propane combustion as a heat source and ammonia decomposition on Ru (for hydrogen production) as an endothermic reaction were chosen. Consistent with our previous work on microburners, it was found that the wall plays an important role in heat recirculation, ignition, and flame stability. Despite operating at the microscale, significant temperature gradients were found in the system, especially in the microchannel carrying out the homogeneous combustion. The high temperatures generated via homogeneous combustion render compact devices operating at short contact times feasible. However, sustained operation is


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Received for review September 15, 2004 Revised manuscript received November 23, 2004 Accepted December 2, 2004 IE0490987

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