High Efficiency Inserted-Plate Coated of Methanol Steam Reformer: PEM Fuel Cell Applications
Methanol can be easily stored and transported and converted to H2 using a reforming reactor that makes it an excellent hydrogen and energy source for fuel cell applications [1-3]
During the last decades, there has been a growing interest on developing technologies taking advantage of clean energy sources. The reduction of atmospheric pollution and, namely, the emission of greenhouse gases have become imperative and, among the new technologies for mitigating these emissions, fuel cells have the ability to efficiently convert chemical into electrical energy. In particular, PEMFCs are zero-pollutants emission systems because they transform the chemical energy of the electrochemical reaction within hydrogen and oxygen into clean electrical power [4, 5]
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Meanwhile, compared to other feedstocks, methanol exploitation shows various advantages as a hydrogen carrier for fuel cell applications and, namely, it can be produced from renewable sources, and the reforming reaction occurs at relatively low temperatures, ca. 240–260 1C
compared to the methane reforming, normally performed at 800–1000 1C  . Methanol steam reforming (MSR) reaction has been seen as a very attractive and promising process for hydrogen production and, according to the scientific literature on the argument, it can be described by the following chemical reactions:
Reaction (1), represents MSR reaction , reaction (2) represents water gas shift reaction and reaction (3) represents the methanol decomposition reaction The reactor design has direct impact on the reaction conversion, but owing to higher technical complexity and manufacturing costs of other designs, the reformers and MRs are normally tubular. However, recent efforts in the area of micro-processing made possible and easier to manufacture other reactor designs and namely well-structured ï¬‚at micro-reactors. A micro-reactor is defined as a device that contains micro structured features, with a sub-millimeter dimension, in which chemical reactions are performed in a continuous manner They present advantages compared to conventional ones such as higher surface-to-volume ratio, smaller mean distance of the specific ï¬‚uid volume to the reactor walls, better heat and matter transfer properties and ï¬‚ow patterns that fit with the reaction needs. Furthermore, ï¬‚at reformers are suitable for stack integration with fuel cells. Packed-bed micro-reactors require well-define catalyst particles, with regular shape and much smaller than the internal dimensions of the micro-channels, which is a problem for most of commercial catalysts .
Many studies have been developed to explore the advantages of micro/mini-reactors to produce hydrogen through MSR The design of a reactor targets the maximization of the conversion and selectivity at the lowest costs and its performance is inï¬‚uenced by the ï¬‚ow pattern, velocity profile, pressure drop and heat transfer, so all these aspects must be considered [ 9 ] . For conducting MSR reaction, most of the used reactor designs are rectilinear channels, pin-hole, coil-based and radial (Fig. 2). Coil-based reactor designs allow high conversions, but impose a significant pressure drop penalty, which may be a limitation for compact applications C . In the other hand, the rectilinear channel designs exhibit a small-pressure drop, but the conversion is low due to uneven mass distribution and is affected by the Reynolds number . Yet, by adjusting the channels width  or by imposing a pressure drop at the channels entrance even distributions on rectilinear channel designs can be obtained, improving the methanol conversion [ 10 ] .
Conventional packed-bed reactor has disadvantages, such as hot spots, delays in start-up, and mass and heat transfer limitations. For micro-scale reactor, the pressure drop is somewhat higher due to space constraints of channels being blocked with catalysts. A micro-pump used in portable applications may not be capable of overcoming a high pressure drop. On the other hand, microchannel reactor offers advantages, such as fast heating and cooling, large surface-area-to volume ratios, and less energy input . Some research has shown that wall-coated reactor performs better than packed bed reactor for SRM reaction [11, 12].
Firstly, 200 ml deionized water was heated to 80ËšC and then metal nitrates of Cu, Zn and Fe (to prepare CuZnFe) were dissolved into the water until get the 0.2M solution. This solution was added to the heated water (80ËšC) over stirrer under 350 rpm. To control and adjust pH around 7, the precipitation agent of 0.5M Na2CO3 was used. The obtained precipitates were aged at 60 ËšC for 2 h under vigorous stirring. Afterward, the solids were filtered and washed with warm deionized water for several times and dried at 110 ËšC for 12 h. then, the dried powder was calcined in a furnace at 350 ËšC for 4h.
Catalyst slurry preparation
PVA (87-90% hydrolyzed, average mol wt 30,000-70,000,SIGMA-ALDRICH) was added to deionized water first, and stirred at 70 C until totally dissolved, then cooled at room temperature. As-synthesized, high-performance CuZnFe catalyst (10 wt.% catalyst) was added into the PVA solution (0.5 wt.%, 1 wt.%, 2 wt.% PVA) as-prepared. The catalyst slurry was kept in the ultrasonic bath for 1 h.
Catalyst coating by electrophoretic deposition on stainless steel plates
To prepare a regular, controlled catalytic layer, a well-stable suspensions of the catalyst powder are necessary which depend on the particle size, solvent characteristics and additional agents. To do this, the powder were milled to get a uniformity of particle size distribution below 40 micron, at least. Isopropanol was selected as solvent due to lower conductivity limiting solvent transportation than of the aqueous ones. Moreover, presence of aqueous base cause water electrolysis following poor coverage of catalyst. Electrophoretic deposition was performed at constant voltage (140 V) using a power supply unit (SPS-900NP-Navasanpardaz). The stainless steel (AISI-304) palates (7.04 cm2) were used as electrodes in the EPD bath. To cover both sides of the plate, a system including three electrodes was selected which, were mounted at a distance of 15 mm in a cell with a total volume of 120 ml. slurry of 72 g/L of catalyst in isopropanol and 1 g/L of PEI as binder was prepared through 15 min strongly stirring and then, 15 min signification in an ultrasonic bath (1200M-Soltec). The time of 4 min as coating time was fixed for all samples. After coating, the plates were dried in room temperature and then calcination at 350 C for 2 h.
Method and materials
According to the procedure in fig. 1, a series of CuZnFe slurries and samples were prepared. The synthesized powder was milled at 250 rpm for 10 min (due to initial fine powder structure short milling time was chosen) by a milling apparatus. It make a stable behavior of the slurry and homogenous final layer deposited.
Results and Discussion
After the coating procedure, the changing structure of the CuZnFe catalyst, the operating parameters and micro-channel characterization affect the efficiency of hydrogen in the output. In this study, explored a number of vital parameters characterization, including channeling arrangement and shape, coating methods and the efficiency of hydrogen production.
Design and micro-channel arrangement optimization cause select an appropriate structure limited by performance efficiency, constant cost like startup, Fabrication, and variable costs (Catalyst stability, coated layer quality, appropriate life time). Regardless of the constant costs, overall state and changing catalytic plates while the plates have been channelized, replacement and re-channelizing, coating procedure should be more difficult. Although, chein et. Al.  adopt three types reformer including the microchannel, the plain channel and the inserted catalyst layer while utilizing channeling over cover plate is different. All proposed structure of a square microchannel have been shown below……….
In this research, flexibility and replacing of an aged micro-reformer has been emphasized, an appropriate configuration with the minimum of fabrication cost through a recoverable coating procedure, minimum catalyst consuming and high efficiency has been adopted. A treated stainless steel as active layer was coated by the high efficiency CuZnFe catalyst which coated in an electrophoretic bath under a high controlling ability condition. The EPD producer will be discussed elsewhere, in detail. According to the optimum condition of a constant parameters, type of micro-channels, length and stability have been investigated.
The reactors were designed as flat-plate composed of stainless steel 304 plates. The microstructures were introduced by CNC technology. The both cover plate were machined to cover the active catalytic layer and increasing area to volume concept. The active layers were coated by home-made synthesized high efficiency steam reforming catalyst. Subsequently, the housing sealed by graphite gasket and bolts. The size and number of channels are summarized in table 1.
Table 1. Summary of the dimensions of the reactor: the channel dimensions are only provided for the reformate side of the plates
The test plates and reformer design details are shown in fig. 1. On the top side of the cover plate, as shown in fig. 1(a), microchannel were patterned zig-zag path to reduce pressure drop along the path, holding high activity of the catalyst through the contact time increasing.(ref) the feed methanol-water mixture is sent to the vaporizer section including an inert alumina granule packed developing the heat transfer, and then, collected in an triangle manifold in the reformer inlet to obtain a more uniformly distributed flow. The design of the reformer channel (as shown in fig) leads splitting-jointing in an alternatively manner. In order to identify the reactant flow pattern effect on the reforming performance, the reformer channels shown in fig.1 were investigated.
The results of coating procedure, have been shown in fig 2. For all the plates, the same catalyst loading of 25mg was coated.
A schematic diagram of the experimental setup is shown in fig. 3. The main components of the system are reactants, micro-reformer and products. To provide the heat of the reaction an electrical furnace was used controlled by a TIC. The feedstock system including methanol-water in liquid phase was sent by a syringe pump (model nnn) in a designed feed composition and determined S/C= 1.3 in all the tests. The reformed gas stream was then sent to a cold trap to eliminate the unreacted liquid phase. In continues, the rest of the gases was conducted to a gas chromatography equipped via two columns of carboxen and hysep Q (model shimadzu GC-8A). Based on the achieved fractions and compositions, the methanol conversion, CO selectivity, hydrogen yield and stability of the active layer cab be achieved. A K-type thermocouple connected at the end of the holes which designed to measure temperature of determined spots along the axis, default of the measurement is based on the central point of the reformer for all the tests.
Results and Discussion
50 mm parallel microchannel
Fig. 4 and fig. 5 display the performance of the microchannels according to the length. The investigation was performed under different temperature and micro channel types. It can be observed that the methanol conversion rates with various lengths is no different although the magnitude of this depends on the micro channel arrangement and for the direct parallel channels is more obvious. In the experiments, the effect of reaction temperature could overcome length and full conversion of methanol was achieved, finally. For the zig zag type, by variation in length in a range of 20 to 50 mm, no difference was observed. The arrangement cause more efficiency in reduction of surface to volume ratio. Actually the need of a sufficient contact time of the reactants on the catalytic active sites is necessary for a satisfying conversion. In the zag-zag type the contact time will increased and the at least of the sufficient length of full conversion is about 25mm. other lengths below those was not possible due to the limitation of the mass flow controller. On the other hand, being endothermic of MSR, the least requirement heat of reaction can be supplied by increasing of the contact time on the cover plate which is contacted to the heating source, consequently following the micro channel length. About the CO content, however, due to being full conversion in considered lengths, there is no significant different among the cases. Higher conversion can cause producing some more CO in the outlet. On the other hand, sufficient existing catalyst affect the CO production due to being active rWGS in lower loading of the catalyst. (ref).
Felani et al. [ ] proposed that a novel channel arrangement with a certain sizing could guarantee better flow distribution, higher contact time being accessible of reaction and heating, and lower pressure drop than those of conventional ones. The proposed model can increase the methanol conversion. Fig. 6 shows the illustration of the micro-reactor with such zig-zag microchannel. The performances of parallel microchannels and that of zig-zag microchannels are compared by the present experiment. The micro-reactor in this study is different from that of chein et al. . In chein’s paper, the reactor only has simple inserted plate without channelizing. It is noticed that in this experiment novel catalyst of CuZnFe has been used and the goal of this experiment is not the comparison of the cases. Advantages of easily replacing, lower costs of recoating and flexibility managing of the fuel processor. Fig. 7 shows the variation of the methanol conversion with the temperatures. It can be found that with the increase of the temperature, the methanol conversion decrease. The conversion in zig-zag microchannels were higher than those in parallel microchannels. When the temperature is high, methanol conversion in zig-zag microchannels could be 20% higher than that in parallel microchannels. As mentioned before, zig-zag microchannels induce potential of providing more contact time to react under the help of catalysts, thereby enhancing the methanol conversion.
In this paper, the experiment of methanol steam reforming were performed in a micro-reformer coated with novel CuZnFe catalyst. The following conclusion can be achieved.
1. The impacts of reaction temperature, gas hourly space velocity, H2O-to-CH3OH molar ratio and catalyst stability were also investigated in this stainless micro-reformer. Condition selection of optimum operation can be achieved by these useful guidance.
2. Micro-reactor with two types microchannel arrangement, including zig-zag path and parallel microchannels, different lengths of 15, 30 and 60 mm were evaluated. It is found that methanol conversion in microchannels with zig-zag path are much higher than that of parallel path. In addition, since zig-zag path cause more contact time of the reactants on active sites of catalyst, there is no different of methanol conversion with length variation, while in the parallel path, it was obvious that higher length leads to more methanol conversion.
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