Hydrogen production with integrated microchannel fuel processor for portable fuel cell systems

An integrated microchannel methanol processor was developed by assembling unit reactors, which were fabricated by stacking and bonding microchannel patterned stainless steel plates, including fuel vaporizer, heat exchanger, catalytic combustor and steam reformer. Commercially available Cu/ZnO/Al₂O₃ catalyst was coated inside the microchannel of the unit reactor for steam reforming. Pt/Al₂O₃ pellets prepared by ‘incipient wetness’ were filled in the cavity reactor for catalytic combustion. Those unit reactors were integrated to develop the fuel processor and operated at different reaction conditions to optimize the reactor performance, including methanol steam reformer and methanol catalytic combustor. The optimized fuel processor has the dimensions of 60 mm × 40 mm × 30 mm, and produced 450sccm reformed gas containing 73.3% H₂, 24.5% CO₂ and 2.2% CO at 230–260 °C which can produce power output of 59 Wt.     

Design of a methanol reformer for on-board production of hydrogen as fuel for a 3 kW High-Temperature Proton Exchange Membrane Fuel Cell power system

The method of Computational Fluid Dynamics is used to predict the process parameters and select the optimum operating regime of a methanol reformer for on-board production of hydrogen as fuel for a 3 kW High-Temperature Proton Exchange Membrane Fuel Cell power system. The analysis uses a three reactions kinetics model for methanol steam reforming, water gas shift and methanol decomposition reactions on Cu/ZnO/Al₂O₃ catalyst. Numerical simulations are performed at single channel level for a range of reformer operating temperatures and values of the molar flow rate of methanol per weight of catalyst at the reformer inlet. Two operating regimes of the fuel processor are selected which offer high methanol conversion rate and high hydrogen production while simultaneously result in a small reformer size and a reformate gas composition that can be tolerated by phosphoric acid-doped high temperature membrane electrode assemblies for proton exchange membrane fuel cells. Based on the results of the numerical simulations, the reactor is sized, and its design is optimized.     

Thermal analysis of a micro tubular reactor for methanol steam reforming by optimizing the multilayer arrangement of catalyst bed for the catalytic combustion of methanolr

Packed bed tube reactors are commonly used for hydrogen production in proton exchange membrane fuel cells. However, the hydrogen production capacity of methanol steam reforming (MSR) is greatly limited by the poor heat transfer of packed catalyst bed. The hydrogen production capacity of catalyst bed can be effectively improved by optimizing the temperature distribution of reactor. In this study, four types of reactors including concentric circle methanol steam reforming reactor (MSRC), continuous catalytic combustion methanol steam reforming reactor (MSRR), hierarchical catalytic combustion methanol steam reforming reactor (MSRP) and segmented catalytic combustion reactor with fins (MSRF) are designed, modeled, compared and validated by experimental data. It was found that the maximum temperature difference of MSRC, MSRR, MSRP and MSRF reached 72.4 K, 58.6 K, 19.8 K and 11.3 K, respectively. In addition, the surface temperature inhomogeneity U_f and CO concentration of the MSRF decreased by 69.8% and 30.7%, compared with MSRC. At the same reactor volume, MSRF can achieve higher methanol conversion rate, and its effective energy absorption rate is 4.6%, 3.9% and 2.6% higher than that of MSRC, MSRR and MSRP, respectively. The MSRF could effectively avoid the influence of uneven temperature distribution on MSR compared with the other designs. In order to further improve the performance of MSRF, the influences of methanol vapor molar ratio, inlet temperature, flow rate, catalyst particle size and catalyst bed porosity on MSR were also discussed in the optimal reactor structure (MSRF).     

Heat transfer effects on the methanol-steam reforming with partially filled catalyst layers

The heat transfer effects on a methanol-steam reforming (MSR) process are numerically investigated using a circular reformer with a partially (referred to as the wall-coated reformer) or entirely filled catalyst layer (referred to as the packed-bed reformer). The catalyst layer is formed by packing CuO/ZnO/Al₂O₃ catalyst particles with a certain porosity and permeability. The fluid flow characteristics are strongly affected by the catalyst-layer thickness. As a consequence, the heat and mass transfer also depend on the catalyst-layer thickness. Under heat supplied by wall heat flux conditions, it is found that higher reforming temperature can be obtained for the wall-coated reformer compared with the packed-bed reformer. This results in the wall-coated reformer having a better methanol conversion efficiency compared with the packed-bed reformer. Based on our study, the minimum heat transfer coefficient, maximum methanol conversion and maximum carbon monoxide production are obtained when the catalyst-layer thickness is 90% of the reformer radius.     

Numerical simulation of configuration and catalyst-layer effects on micro-channel steam reforming of methanol

A micro-reactor with eight non-parallel channels is proposed to improve the performance of micro-channel steam reforming of methanol. The widths of some channels in the micro-reactor vary gradually along the reactor length direction. The Zn-Cr/CeO₂-ZrO₂ catalyst is coated in the reformer with a certain porosity and permeability. The effects of micro-reactor structures and catalyst-coated manners on several factors are studied, including temperature distributions, velocity distributions, reactant concentrations and the methanol conversion rate. The results indicate that such a structure with a certain entrance inclination angle and channel inclination angle guarantees flow distribution uniformity in each reforming channel. Flow distribution uniformity is conducive to the increase of the methanol conversion rate. Besides, in order to measure strengths and weaknesses of different catalyst-coated manners, a wall-coated reformer and a packed-bed reformer are studied respectively. It is found that compared to the packed-bed reformer, the temperature and the methanol conversion rate in wall-coated reformer are far higher. It is necessary to find an optimal catalyst thickness that is able to reduce the CO concentration because the catalyst thickness can affect CO concentration in the product gases indirectly. The optimal inclination angles and the catalyst thickness are proposed based on the simulating results.     

Hydrogen production from steam reforming using an indirect heating method

In this work, a methanol steam reforming (MSR) reactor was operated using an indirect heating method. A thermal circuit was constructed between the MSR reactor and the electrical heater to supply the heat required for the endothermic reaction, and deionized water was used as the heat transfer medium (HTM). The MSR reactors featured a shell-and-tube type design to operate at high pressures. A Cu/Zn catalyst was installed on the tube side, and HTM was supplied to the shell side. To improve the heat transfer performance, the heat transfer area between the shell and tube was increased from 598 to 1117 cm². Because the MSR reactor had a sufficient heat exchange area corresponding to the catalytic reaction rate, the heat exchange area had little effect on methanol conversion. However, the heat exchange area had a greater effect on the performance because the operating temperature of reactor was lower. Under the same operating temperature conditions, the MSR reactor operated under the indirect heating method showed relatively higher methanol conversion than the MSR reactor operated in an electric furnace because of the effective heat transfer by the latent heat of saturated steam. The MSR reactor based on the indirect heating method was continuously operated at 250 °C for 72 h to verify characteristic start-up and operation. The results showed that the MSR reactor could be operated at a constant temperature; however, low methanol conversion at low operating temperatures led to slow catalyst degradation. In addition, the MSR reactor required more than 2 h for initial start-up and for restart after emergency shutdown because the HTM needed to be evaporated and pressurized to the target pressure.     

Conceptual design of an integrated thermally self-sustained methanol steam reformer – High-temperature PEM fuel cell stack manportable power generator

We have investigated the concept of an integrated system for small, manportable power units. The focus of this study is the direct thermal coupling of a methanol steam reformer (MSR) and a high-temperature proton exchange membrane fuel cell (HT PEMFC) stack. A recently developed low-temperature (LT) MSR catalyst (CuZnGaO_x) was synthesized and tested in a designed reforming reactor. The experimental data show that at 200 °C the complete conversion of methanol is achievable with a hydrogen yield of 45 cm³ min^?1 g_CAT^?1. An experimental setup for measuring the characteristics of the integrated system was designed and used to measure the characteristics of the two-cell HT PEMFC stack. The obtained kinetic parameters and the HT PEMFC stack characteristics were used in the modeling of the integrated system. The simulations confirmed that the integrated LT MSR/HT PEMFC stack system, which also includes a vaporizer, can achieve a thermally self-sustained working point. The base-case scenario, established on experimental data, predicts a power output of 8.5 W, a methanol conversion of 98.5%, and a gross electrical efficiency (based on the HHV) of the system equal to 21.7%. However, by implementing certain measures, the power output and the electrical efficiency can readily be raised to 11.1 W and 35.5%, respectively.     

Novel micro-channel methane reformer assisted combustion reaction for hydrogen production

A metal catalyst-containing, 80 ml, micro-channel reactor (MCR) with a section dedicated to combustion reaction was investigated for the potential application of on-board methane steam reforming (MSR) to hydrogen production. The metal catalyst was introduced into the MCR as a shape of a thin plate that was diffusion-bonded with the other micro-channel plates. The combustion reaction was performed on the other side of the MCR for direct provision of the necessary heat for the endothermic MSR and for miniaturizing the system volume. In the MCR, both the methane conversion and the hydrogen production rate are extremely high compared with those of the equilibrium under atmospheric pressure. The required heat of reaction is successfully provided by the combustion of either hydrogen or the methane mixture on the other side of the MCR without the need for any heating cartridges. This novel micro-channel reformer is suitable for application as a compact fuel processor due to its production of hydrogen-rich syn-gas, small volume, simple catalyst loading and use of an active and easily stackable catalyst.     

Optimal monolithic configuration for heat integrated ethanol steam reformer

A design concept for optimal design of monolith catalyst is presented through modeling of transport–kinetic interactions in a monolith catalyst. We argue that reactors employing monolithic catalysts should be based on its optimal choice of geometry. In line with that argument, we present a thorough analysis of the geometrical parameters influencing the performance of non-isothermal reactor operation. In this study, an optimal monolith configuration is estimated to be a combination (d_h, t_w) of (0.9 mm, 0.2 mm) for a compact ethanol reformer to produce hydrogen for portable applications where maximum volumetric reactor activity exists. A three-dimensional modeling framework is developed for the resulting optimal monolithic catalyst design that couples the reforming section with a suitable heat source in a recuperative way. As a result, greater ethanol conversion is obtained from the monolith channels near the periphery of the block. The coupling with combustion could predict the formation of cold and hot spots inside the reactor, their nature being dependent on the flow configuration. Further, the effect of altering the feed inlet operating conditions over the variation of ethanol conversion and temperature inside the reactor is also analyzed. The increase in reforming inlet velocity decreases the outlet conversion and shifts the cold spot, forward and deeper in co-flow configuration. The decreasing inlet feed temperature enhances the transfer of heat, eliminating the cold spot.     

Microfibrous structured catalytic packings for miniature methanol fuel processor: Methanol steam reforming and CO preferential oxidation

The microfibrous structured catalytic packings for miniature fuel processor consisting of a methanol steam reformer and a subsequent CO cleanup train has been investigated experimentally. A highly void and tailorable sinter-locked microfibrous carrier consisting of 3.5 vol% 8 μm diameter Ni-fibers is used to entrap 35 vol% 150-250 μm catalyst particulates for both methanol steam reforming (MSR) and CO preferential oxidation (PROX). We demonstrate a microfibrous entrapped Pd-ZnO/Al₂O₃ catalyst packings for high efficiency hydrogen production by the MSR reaction. The use of microfibrous entrapment technology significantly enhances the catalyst utilization efficiency by a 4-fold improvement of the weight hourly space velocity (WHSV), compared to the single Pd-ZnO/Al₂O₃ particulates as keeping the methanol conversion at >98%. The microfibrous entrapped Pt-Co/Al₂O₃ catalyst packings can drive the CO from 2% down to <50 ppm at 150 °C with O₂/CO ratio of 1 using a gas hourly space velocity (GHSV) of 32,000 h⁻¹. Finally, a prototype fuel processor system integrating MSR reformer and CO PROX train is demonstrated as three reactors in series. Such test rig is capable of producing roughly 1700 standard cubic centimeter per minute (sccm) PEMFC-grade H₂ (equivalent to ∼163 W of electric power) in a longer-term test, in which the MSR reactor is operated at 300 °C using a methanol/water (1/1.1, mole) mixture WHSV of 9 h⁻¹ and CO PROX reactors at 150 °C using an O₂/CO molar ratio of 1.3, respectively. In the test of this prototype system, MSR reactor delivers >97% methanol conversion throughout the entire 1200-h test; the CO cleanup train placed in line after 800-h MSR illustrates the capability to decrease the CO concentration from ∼3.5% to ∼1% at PROX-1 and then to less than 20 ppm at PROX-2 until to the end of test.     

Design and test of a miniature hydrogen production reactor integrated with heat supply,fuel vaporization,methanol-steam reforming and carbon monoxide removal unit

This study presents a designed and tested integrated miniature tubular quartz-made reactor for hydrogen (H₂) production. This reactor is composed of two concentric tubes with an overall length of 60 mm and a diameter of 17 mm. The inner tube was designed as the combustor using Pt/Al₂O₃ as the catalyst. The gap between the inner and outer tubes is divided into three sections: a liquid methanol-water vaporizer, a methanol-steam reformer using RP-60 as the catalyst and a carbon monoxide (CO) methanator using Ru/Al₂O₃ as the catalyst. The experimental measurements indicated that this integrated reactor works properly as designed. The methanol conversion, hydrogen production rate and CO concentration were found to increase with an increasing methanol/air flow rate in the combustor and decreases with an increasing methanol/water feed rate to the reformer. The methanator experimental results indicated that the CO conversion and H₂ consumption can be enhanced by increasing the Ru loading. It was also found that the CO methanation depends greatly on the reaction temperature. With a higher reaction temperature, the CO methanation, carbon dioxide (CO₂) methanation, and reversed water gas shift reactions took place simultaneously. CO conversion was found to decrease while H₂ consumption was found to increase. At a lower reaction temperature both the CO conversion and H₂ consumption were found to increase indicating that only CO methanation took place. From the experimental results the maximum methanol conversion, hydrogen yield, and CO conversion achieved were 97%, 2.38, and 70%, respectively. The actual lowest CO concentration and maximum power density based on the reactor volume were 90 ppm and 0.8 kW/L, respectively.     

Comparison between 1D and 2D numerical models of a multi-tubular packed-bed reactor for methanol steam reforming

The hydrogen-rich gas produced in-situ by methanol steam reforming (MSR) reactions significantly affects the performance and endurance of the high-temperature polymer electrolyte membrane (HT-PEM) fuel cell stack. A numerical study of MSR reactions over a commercial CuO/ZnO/Al₂O₃ catalyst coupling with the heat and mass transfer phenomena in a co-current packed-bed reactor is conducted. The simulation results of a 1D and a 2D pseudo-homogeneous reactor model are compared, which indicates the importance of radial gradients in the catalyst bed. The effects of geometry and operating parameters on the steady-state performance of the reactor are investigated. The simulation results show that the increases in the inlet temperature of burner gas and the tube diameter significantly increase the non-uniformity of radial temperature distributions in reformer tubes. Hot spots are formed near the tube wall in the entrance region. The hot-spot temperature in the catalyst bed rises with the increase in the inlet temperature of burner gas. Moreover, the difference in simulation results between the 1D and 2D models is shown to be primarily influenced by the tube diameter. With a methanol conversion approaching 100% or a relatively small tube diameter, the simplified 1D model can be used instead of the 2D model to estimate the reactor performance.     

Experimental study on partial oxidation of methane to produce hydrogen using low‐temperature plasma in AC Glidarc discharge

A reformer using low‐temperature plasma was designed and developed for hydrogen production. The reformer has three electrodes and uses AC gliding arc discharge. A reference condition, which is the highest hydrogen production, has a O₂/C ratio of 0.45, input flow rate of 4.9 l min^?1 and power supply of 1 kW. And the methane conversion rate, the high hydrogen selectivity and the reformer efficiency were 69.2, 77.8 and 35.2%, respectively. To investigate reforming characteristics, parametric studies were achieved for the gas components ratio, a gas flow rate, a reactor temperature, an input electric power and catalyst addition effect. The results are as follows: The gas components ratio was an important factor, which had maximum value. When the gas flow rate, the reactor temperature and the electric power were increased, the methane conversion rate and the hydrogen concentration also increased. Copyright © 2008 John Wiley & Sons, Ltd.     

A study on the integration of micro‐reformer and high‐ temperature PEM fuel cell

This study presented an integration platform for a methanol reformer and high‐temperature proton exchange membrane fuel cell (PEMFC). The methanol micro‐reformer was combined with the catalytic reaction section and reforming section, whereas the catalytic reaction section with Pt catalysis maintained the constant temperature environment for a reforming process. SRM reforming results showed that 74 to 74.9% hydrogen and 23.5 to 25.7% of carbon dioxide in the mixture product, and less than 2% of carbon monoxide, was produced. Using the reforming product of low carbon monoxide concentration and the highest methanol conversion rate, a micro reformer link with a fuel cell integration experiment was performed. Results showed a high temperature PEMFC with 3 to 4 W power output under methanol flow rates of 15 ml/hr. Due to the lower hydrogen pressure supplied from the micro reformer, the fuel cell power output may become unstable. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20322     

A comparison of hydrogen and methanol production in a thermally coupled membrane reactor for co‐current and counter‐current flows

This work considers three concentric tube reactors to prepare pure hydrogen, especially applicable in fuel cell technologies, with zero CO₂ emission. Hydrogen and methanol production rates are compared in a thermally coupled exothermic and endothermic reactor for co‐current and counter‐current modes. Synthesis of methanol is coupled with dehydrogenation of cyclohexane as a high content hydrogen carrier (7.1 wt%). The efficient coupling of exothermic and endothermic reactions increases the profitability of operation of the reactor, reduces the size of reactor and decreases the operational and capital costs. By inserting a hydrogen‐perm selective membrane into the reactor configuration, hydrogen can permeate selectively into the membrane, and hence, the third tube receives hydrogen. The simulation results are compared with the corresponded results for an industrial methanol fixed‐bed reactor, which operates under the same feed conditions. The influence of some operating variables is investigated on methanol and hydrogen yields during the performance of reactor. The results show higher methanol conversion, as the same as conventional reactor, and hydrogen for co‐current flow. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermal resistance effect on methanol-steam reforming performance in micro-scale reformers

We numerically investigate hydrogen production based on methanol-steam reforming (MSR) using a micro-scale cylindrical packed bed reformer. The reformer wall is included in the physical model. The heat required for the reforming reaction is supplied either internally using a heating rod placed along the center of the reformer or externally by a heat flux applied at the reformer outer wall. Our results show that the thermal resistance from the heat source to the reformer environment plays an important role in the reformer performance. This thermal resistance depends on the reformer geometry, wall material and heat transfer coefficients inside the catalyst bed and outside the reformer. Based on our numerical results, it is suggested that better methanol conversion and hydrogen yield can be obtained using reformer wall material with low thermal conductivity and thin thickness. For both internal and external heating under the same heat rate supply, no significant difference in reformer performance was found.A water gas shift (WGS) reaction model was included in the present numerical model. In the reformer low-temperature zone the forward WGS reaction was clearly demonstrated, resulting in a decrease in carbon monoxide (CO) selectivity. In the high temperature zone the backward WGS reaction was also clearly demonstrated in which CO selectivity increases with the increase in temperature. For both internal and external heating under the same heat rate supply, our results indicated that CO selectivity is about thirty times lower when the WGS reaction is neglected.     

Micro methanol reformer combined with a catalytic combustor for a PEM fuel cell

This paper presents the development of a micro methanol reformer for portable fuel cell applications. The micro reformer consists of a methanol steam reforming reactor, catalytic combustor, and heat exchanger in-between. Cu/ZnO was selected as a catalyst for a methanol steam reforming and Pt for a catalytic combustion of hydrogen with air. Porous ceramic material was used as a catalyst support due to the large surface area and thermal stability. Photosensitive glass wafer was selected as a structural material because of its thermal and chemical stabilities. Performance of the reformer was measured at various test conditions and the results showed a good agreement with the three-dimensional analysis of the reacting flow. Considering the energy balance of the reformer/combustor model, the off-gas of fuel cell can be recycled as a feed of the combustor. The catalytic combustor generated the sufficient amount of heat to sustain the steam reforming of methanol. The conversion of methanol was 95.7% and the hydrogen flow of 53.7 ml/min was produced including 1.24% carbon monoxide. The generated hydrogen was the sufficient amount to operate 4.5 W polymer electrolyte membrane fuel cells.     

A transient study of double-jacketed membrane reactor via methanol steam reforming

A non-isothermal unsteady-state model was established to simulate methanol steam reforming using a double-jacketed Pd membrane reactor. At steady state, a self-sustained membrane reactor was achieved by the oxidation of residual methanol and hydrogen from reformer for endothermic steam reforming. The molar fractions of species and reformer temperature were analyzed under co-current operation between oxidation and reformer sides. The start-up of reformer was simulated under two conditions: (1) The catalyst temperature was lower than the influent temperature and (2) The catalyst temperature was higher than influent temperature. Condition 1 yielded higher methanol conversion and reformer temperature than condition 2 at steady state. Moreover, the instability of species can be minimized on condition 1 during start-up. The fluctuation of membrane reactor at steady state was also studied. Two strategies were compared to analyze the reformer response when temporary extra hydrogen was required. The results showed that increasing inlet methanol outperformed increasing reformer temperature.     

Numerical predictions on thermal characteristic and performance of methanol steam reforming with microwave-assisted heating

Microwave irradiation is an effective route to trigger methanol steam reforming (MSR) for hydrogen production because of the double absorption of microwaves by both the reagents and the catalyst. To recognize the thermal characteristic and performance of MSR in an environment with microwave irradiation, a numerical method is employed to predict the coupling phenomena of electromagnetic field, flow field and chemical reactions. Methanol decomposition (MD) is also regarded while MSR proceeds. Two sets of complex relative permittivity are established to account for the transmission of electromagnetic waves to heat reactants in the non-porous zone and the porous zone. Moreover, the effects of heat loss, microwave power and reagent flow rate on MSR are taken into account. The predictions suggest that heat loss is an important factor when the appropriate models with microwave-assisted heating are developed. A higher power is conducive to the performance of MSR, resulting from more heat generated in the reaction tube. At a given microwave power, heat generation due to microwave irradiation is independent of Reynolds number. As a result, increasing Reynolds number lessens methanol conversion and H₂ yield, as a consequence of more reagents passing through the catalyst bed. However, it is of interest that there exists an optimal Reynolds number for MSR to consume heat stemming from microwave irradiation.     

Simultaneous production of two types of synthesis gas by steam and tri‐reforming of methane using an integrated thermally coupled reactor: mathematical modeling

In this work, tri‐reforming and steam reforming processes have been coupled thermally together in a reactor for production of two types of synthesis gases. A multitubular reactor with 184 two‐concentric‐tubes has been proposed for coupling reactions of tri‐reforming and steam reforming of methane. Tri‐reforming reactions occur in outer tube side of the two‐concentric‐tube reactor and generate the needed energy for inner tube side, where steam reforming process is taking place. The cocurrent mode is investigated, and the simulation results of steam reforming side of the reactor are compared with corresponding predictions for thermally coupled steam reformer and also conventional fixed‐bed steam reformer reactor operated at the same feed conditions. This reactor produces two types of syngas with different H₂/CO ratios. Results revealed that H₂/CO ratio at the output of steam and tri‐reforming sides reached to 1.1 and 9.2, respectively. In this configuration, steam reforming reaction is proceeded by excess generated heat from tri‐reforming reaction instead of huge fired‐furnace in conventional steam reformer. Elimination of a low performance fired‐furnace and replacing it with a high performance reactor causes a reduction in full consumption with production of a new type of synthesis gas. The reactor performance is analyzed on the basis of methane conversion and hydrogen yield in both sides and is investigated numerically for various inlet temperature and molar flow rate of tri‐reforming side. A mathematical heterogeneous model is used to simulate both sides of the reactor. The optimum operating parameters for tri‐reforming side in thermally coupled tri‐reformer and steam reformer reactor are methane feed rate and temperature equal to 9264.4 kmol h^?1 and 1100 K, respectively. By increasing the feed flow rate of tri‐reforming side from 28,120 to 140,600 kmol h^?1, methane conversion and H₂ yield at the output of steam reforming side enhanced about 63.4% and 55.2%, respectively. Also by increasing the inlet temperature of tri‐reforming side from 900 to 1300 K, CH₄ conversion and H₂ yield at the output of steam reforming side enhanced about 82.5% and 71.5%, respectively. The results showed that methane conversion at the output of steam and tri‐reforming sides reached to 26.5% and 94%, respectively with the feed temperature of 1100 K of tri‐reforming side. Copyright © 2013 John Wiley & Sons, Ltd.