Comparison of conventional and metal fiber burners in a compact methane reformer using CFD modeling

Compact reformers can be used to produce hydrogen for fuel-cell automobiles. The heat of the mehane seam reforming reaction is provided by methane burning. Generally, conventional burners have been used in combustion chambers. The Computational Fluid Dynamic (CFD) approach was used for the comparison of conventional burners with metal fiber burners and their locations for the first time. The rate of steam reforming reactions and methane combustion reactions were introduced to the CFD model and the Finite Rate/Eddy Dissipation model was used for reactions on the reforming and combustion sections. After validation of the compact reformer results by available experimental data, metal fiber was modeled using the porous-jump interior boundary condition. The results show that the best burner position for the metal fiber is the Bottom (near the catalyst) and for the conventional burner is the Top (far from the catalyst). The results show that the conventional burner in both the Middle and Bottom positions leads to an increase in the reaction zone temperature above 1200 K, which is higher than the catalyst tolerance, but placing a simple burner on the Top of the reactor does not have an out-of-range temperature problem. The hydrogen mass yield for a conventional burner at the Top position is 27.75% relative to methane. Due to the thermal uniformity in the metal fiber burner, the temperature does not exceed the catalyst limitation in the three positions (Top, Middle, and Bottom). The metal fiber burner at the Bottom of the combustion chamber shows the best performance with a hydrogen mass yield of 40.82%. The results indicate that metal fiber burners can distribute the flame more uniformly than conventional burners and increase the available heat for the reformer side.     

Methane steam reforming in a novel ceramic microchannel reactor

Microchannel heat exchangers and reactors can deliver very high performance in small packages. Such heat exchangers are typically fabricated from aluminum, copper, stainless steel, and silicon materials. Ceramic microchannel reactors offer some significant advantages over their metallic counterparts, including very-high-temperature operation, corrosion resistance in harsh chemical environments, low cost of materials and manufacturing, and compatibility with ceramic-supported catalysts. This work describes a ceramic microchannel reactor that achieves process intensification by combining heat-exchanger and catalytic-reactor functions to produce syngas. A complete computational fluid dynamics (CFD) model as well as a geometrically simplified hybrid CFD/chemical kinetics model is used in conjunction with experimentation to examine heat transfer, fluid flow, and chemical kinetics within the ceramic microchannel structure. Heat-exchanger effectiveness of up to 88% is experimentally demonstrated. Reactive heat-exchanger performance for methane-steam reforming reaches 100% methane conversion and high selectivity to syngas at a gas hourly space velocities (GHSV) of 15,000 h⁻¹. Model results agree well with experimental data and provide insight into physical processes underway during reactor operation.     

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.     

Integrated fuel cell APU based on a compact steam reformer for diesel and a PEMFC

This paper presents experimental results of a diesel steam reforming fuel processor operated in conjunction with a gas cleanup module and coupled operation with a PEM fuel cell. The fuel processor was operated with two different precious-metal based reformer catalysts, using diesel surrogate with a sulfur content of less than 2 ppmw as fuel. The first reformer catalyst entails an increasing residual hydrocarbon concentration for increasing reformer fuel feed. The second reformer catalyst exhibits a significantly lower residual hydrocarbon concentration in the reformate gas.     

Design of compact methanol reformer for hydrogen with low CO for the fuel cell power generation

The effect of the heat transfer area and the thermal conductivity of the reactor materials are evaluated with three identical structured reactors having multiple columned-catalyst bed and using three different reactor materials, aluminum alloy, brass and stainless steel. A series of compact methanol reformers are then designed and fabricated with the use of large reactor surface area in catalyst beds and high heat transfer constant to produce hydrogen fuel with 2–4 ppm of CO for the fuel cell (FC) power generation. The same design principle is successfully used for easy scale up of the reactor capacity from 250 L/h to 10,000 L/h. This low CO hydrogen (68–70%) used as the fuel for the fuel cell power generation provides a very competitive cost of hydrogen and electric power, $0.20–0.23/m³ of H₂ and $0.196/KWh, respectively.     

A modelling evaluation of an ammonia-fuelled microchannel reformer for hydrogen generation

Hydrogen production from an ammonia-fuelled microchannel reactor is simulated in a three-dimensional (3D) model implemented via Comsol Multiphysics™. The work described in this paper endeavours to obtain a mathematical framework that provides an understanding of reaction-coupled transport phenomena within the microchannel reactor. The transport processes and reactor performance are elucidated in terms of velocity, temperature, and species concentration distributions, as well as local reaction rate and NH₃ conversion profiles. The baseline case is first investigated to comprehend the behaviour of the microchannel reactor, then microstructural design and operating parameters are methodically altered around the baseline conditions to explore the optimum values. The simulation results show that an optimum NH₃ space velocity (GHSV) of 65,000 Nml g_cat⁻¹ h⁻¹ yields 99.1% NH₃ conversion and a power density of 32 kW_e L⁻¹ at the highest operating temperature of 973 K. It is also shown that a 40-μm-thick porous washcoat is most desirable at these optimum conditions. Finally, a low channel hydraulic diameter (225 μm) is observed to contribute to high NH₃ conversion. Mass transport limitations in the porous-washcoat and gas-phase are negligible as depicted by the Damköhler and Fourier numbers, respectively. The experimental microchannel reactor yields 98.2% NH₃ conversion and a power density of 30.8 kW_e L⁻¹ when tested at the optimum operating conditions established by the model. Good agreement with experimental data is observed, so the integrated experimental-modelling approach developed in this paper may well provide an incisive step toward the efficient design of ammonia-fuelled microchannel reformers.     

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.     

Demonstration of a scaled-down autothermal membrane methane reformer for hydrogen generation

A novel concept for hydrogen generation by methane steam reforming in a thermally coupled catalytic fixed bed membrane reformer is experimentally demonstrated. The reactor, built from three concentric compartments, indirectly couples the endothermic methane steam reforming with the exothermic methane oxidation, while hydrogen is separated by a permselective Pd membrane. The study focuses on the determination of the key operation parameters and understanding their influence on the reactor performance. It has been shown that the reactor performance is mainly defined by the dimensionless ratio of the methane steam reforming feed flow rate to the hydrogen maximal membrane flow rate and by the ratio of the oxidation and steam reforming methane feed flow rates.     

Investigation of a methanol processing system comprising of a steam reformer and two preferential oxidation reactors for fuel cells

Methanol steam reforming is able to produce hydrogen-rich syngas onsite for fuel cells and avoids the problems of hydrogen storage. Nevertheless, CO in the reformate needs to be further removed to ppm level before it can be fed into proton exchange membrane fuel cells. In this study, a methanol processing system consisting of a methanol reformer and two-stage preferential oxidation reactors is developed. The hydrogen production performance and scalability of the reformer are experimentally investigated under various operating conditions. The methanol reformer system shows stable methanol conversion rate and linearly increased H₂ flow rate as the number of repeating unit increases. Methanol conversion rate of 96.8% with CO concentration of 1.78% are achieved in the scaled-up system. CO cleanup ability of the two-stage preferential oxidation reactors is experimentally investigated based on the reformate compositions by varying the operating temperature and O₂ to CO ratios. The results demonstrate that the developed CO cleanup train can decrease the CO concentration from 1.6% to below 10 ppm, which meets the requirement of the fuel cell. Finally, stability of the integrated methanol processing system is tested for 180 h operation.     

Thermal design of methane steam reformer with low-temperature non-reactive heat source for high efficiency engine-hybrid stationary fuel cell system

In hybrid fuel cell systems, the fuel-lean anode-off gas is very useful to improve the system efficiency via additional power generation or utilization of thermal energy for heating up of auxiliary devices. In this study, the thermal energy of the hybrid systems is firstly utilized in homogeneous charge combustion engine for additional power and is then supplied to heat up the external reformer. Different from other hybrid fuel cell systems, it is very difficult to utilize heat energy of exhausted gas from engine due to its low temperature characteristics. This study is concentrated on the computation analysis of external methane steam reformers with engine out exhausted gases. Computational model is validated with experiment and parametric study is conducted. Results show that the temperature uniformity of the longitudinal and radial directions is crucial for the methane conversion efficiency. Additionally, the methane conversion rate also depends on the performance of tube-side heat transfer. When the total methane flow is fixed, the methane conversion rate shows trade-off with increasing steam-to-carbon ratio (SCR). Finally, the sensitivity study shows that heat transfer area and reactor length are dominant parameters for steam reforming with engine out exhausted gases.     

CFD simulation of methane steam reforming in a membrane reactor: Performance characteristics over range of operating window

Methane steam reforming is the most widely used pathway for hydrogen production. In this context, the use of a fixed bed catalytic reactor with a hydrogen-selective membrane is one of the most promising technologies to produce high purity hydrogen gas. In this work, the membrane reactor three-dimensional computational fluid dynamic (CFD) model was developed to investigate the performance. In this model, methane steam reforming global kinetic model has been coupled with the CFD model using User-Defined Function (UDF). Whereas, hydrogen permeation across the membrane is implemented by introducing source and sink formulation. The CFD simulation results were compared to the experimental data, where the developed model successfully captured the experimentally observed trends. We studied the influence of the various operating parameters, as temperature, steam to carbon ratio, sweep gas flow configuration and space velocity on the overall performance. The main observation and attained optimal operation windows from the study was discussed to provide insight into the factors affecting the overall performance.     

Long-term stability at fuel processing of diesel and kerosene

The long-term stability at autothermal reforming of diesel fuel and kerosene was studied using Juelich's autothermal reformer ATR 9.2, which is equipped with a commercial proprietary RhPt/Al₂O₃–CeO₂ catalyst. The experiment was run for 10,000 h of time on stream at constant reaction conditions with an O₂/C molar ratio of 0.47, a H₂O/C molar ratio of 1.9, and a gas hourly space velocity of 30,000 h⁻¹. Kerosene produced via the gas-to-liquid process and diesel fuel synthesized via the bio-to-liquid route were used. Both fuels were almost free of mass fractions of sulfur and aromatics. The trends for the desired main products of autothermal reforming H₂, CO, CO₂, and CH₄ were almost stable when kerosene was used. When the fuel mass flow was switched to diesel fuel however, different modes of catalyst deactivation occurred (active sites blocked by carbonaceous deposits, sintering processes), leading to a decrease in the concentrations of H₂ and CO₂ with a simultaneous increase in the CO content. This paper defines carbon conversion as the decisive criterion for evaluating the long-term stability during autothermal reforming of kerosene and diesel fuel. Carbon conversion was diminished via three different pathways during the long-term experiment. Undesired byproducts found in the gas phase leaving the reactor had the strongest impact on carbon conversion. These byproducts included ethene, propene, and benzene. Furthermore, a liquid oily residue was detected floating on the condensed unconverted mass flow of water. This happened once during the whole experiment. Finally, undesired organic byproducts were dissolved in the mass flow of unconverted water. These were found to be straight-chain and branched paraffins, esters, alcohols, acids, aldehydes, ketones, etc. Nevertheless, at the end of the long-term experiment, carbon conversion still amounted to more than 98.2%.     

Membrane reformer module with Ni-foam catalyst for pure hydrogen production from methane: Experimental demonstration and modeling

Results of experiments and modeling of a compact (800 cm³) membrane reformer module for the production of 0.25–0.30 Nm³/h hydrogen by methane steam reforming are reported. The module consists of a two-sided composite membrane disc with a 50 μm PdAg layer and two adjacent 4 mm thick Ni foam discs (60 ppi). A nickel catalyst and a porous support were deposited on the foam discs to give the final composition of 10%Ni/10%MgO/Ni-foam. Membrane permeability by pure hydrogen was investigated, and coefficients of transverse hydrogen transport across the Ni foam to the membrane in the case of inlet binary N₂H₂ mixture were refined in order to account for concentration polarization effect into the model. Activity of the catalytic discs was measured in a differential laboratory scale reactor at a pressure of 1 bar and temperature of 400–600 °C. Modules were tested at a 8–13 bar pressure of the mixture in the reforming zone and at 1 bar of pure hydrogen under the membrane, H₂O/C = 2.5–3 and a module temperature of 550–680 °C (with and without hydrogen removal). Two modifications of the module were tested: consecutive (I-type) and parallel (II-type) flow of the reaction mixture around two sides of the membrane disc. In order to optimize construction of the module, calculations were made for revealing the effect of thickness of the PdAg membrane layer (5–50 μm), thickness of the Ni foam discs (0.5–8 mm) and temperature (600–700 °C) on the hydrogen output of the module. A comparison of the values obtained in our experiments (>1 MW/m³ and >0.7 kg(H₂)/h/m²) with the literature data reported by other authors showed that the developed modules are promising for practical application as components of a fuel processor section for mobile applications.     

Further development of a microchannel steam reformer for diesel fuel

This paper presents results from the ongoing optimisation of a microchannel steam reformer for diesel fuel which is developed in the framework of the development of a PEM fuel cell system for vehicular applications. Four downscaled reformers with different catalytic coatings of precious metal were operated in order to identify the most favourable catalyst formulation. Diesel surrogate was processed at varying temperatures and steam to carbon ratios (S/C). The reformer performance was investigated considering hydrogen yield, reformate composition, fuel conversion, and deactivation from carbon formation. Complete fuel conversion is obtained with several catalysts. One catalyst in particular is less susceptible to carbon formation and shows a high selectivity.     

Parameter study of a porous solar-based propane steam reformer using computational fluid dynamics and response surface methodology

Solar-driven steam reforming of fossil fuels is a promising renewable method for hydrogen production that reduces emissions compared with traditional approaches such as combustion-based technologies. In the present study, a steady-state computational fluid dynamic (CFD) model is developed to investigate a porous solar propane steam reformer (PSR). P1 approximation for radiation heat transfer is coupled with the CFD model, employing User-Defined Functions (UDFs). Innovative propane steam reformers have received less attention in terms of optimization and sensitivity analysis to improve their performance and efficiency. Hence, the effects of porosity, pore diameter, inlet velocity, solar irradiation flux, inlet temperature, and foam thermal conductivity on the propane conversion, hydrogen production rate, and pressure drop are studied using response surface methodology (RSM). The inlet velocity, solar irradiation flux, and pore diameter are found to be the most influential parameters, among those mentioned, on propane conversion, hydrogen productivity, and pressure drop, respectively. Furthermore, optimization is carried out in order to minimize pressure drop and maximize hydrogen production. The reformer with the 70% propane conversion provides the lowest pressure drop maintaining the same hydrogen productivity compared with 80% and 90% propane conversions.     

Methanol reformer integrated phosphoric acid fuel cell (PAFC) based compact plant for field deployment

Naval Material Research Laboratory (NMRL), based on the firm confidence of her core competence on material development, started an ambitious program on development of fuel cells for various Defense and non-Defense application in early nineties. The primary emphasis of this program is to develop phosphoric acid fuel cell (PAFC) based power plants integrated with hydrogen generators along with other accessories. In the process of development, it is understood that online generation of hydrogen from a liquid fuel is the key to success. Methanol, a liquid fuel, can be reformed easily with few side products and the resultant hydrogen rich reformer gas can be directly fed to a PAFC. Such configuration keeps the basic system simple and free of complicated filters and instrumentation.NMRL has developed a series of catalytic burners with high efficiency as the primary heat transfer source from the hot catalytic surface is based on conduction rather than convection as is done normally. Vaporizer is a coiled arrangement and reformer is hollow sections filled with Cu/Al₂O₃/ZnO catalyst, and the same is integrated with catalytic burners. Such arrangement is modular in nature and each reformer has hydrogen generation capacity of 90 lpm and start-up time is around half an hour. Modular design of reformer reactor allow them to used in different capacity plants such as a 2 kW plant configured with a reformer reactor with two vaporizer and 15 kW plant configured with seven nos. of reformer reactors and seven no. of vaporizer. The waste heat of the fuel cell and the same from the reformer burner flue is used to meet most of the reformer heat load. The catalytic burner of the reformer burns both waste hydrogen and methanol with very little excess air. PAFC being tolerant to CO (up to 1%) can be directly operated with the hydrogen rich reformer gas and the lean gas from the fuel cell is burnt into the reformer system.The raw DC output power is converted into either 100 VDC or 220 V single phase, 50 Hz sinusoidal AC power through appropriate power electronics. These configurations give overall efficiency of the plant to around 35-40 % based on LHV of Hydrogen. A battery bank is also incorporated to cater for the plant start-up and other temporary auxiliary power which get charged from the fuel cell output. Such configuration lead to the development of methanol reformer integrated PAFC based power plants of capacity ranging from 2 kW to 15 kW. The system is designed for continuous power production in the field. These plants are suitable for remote area, distributed power generation and application such as battery charging, domestic load etc.     

Dynamic modeling of a three-stage low-temperature ethanol reformer for fuel cell application

A low-temperature ethanol reformer based on a cobalt catalyst for the production of hydrogen has been designed aiming the feed of a fuel cell for an autonomous low-scale power production unit. The reformer comprises three stages: ethanol dehydrogenation to acetaldehyde and hydrogen over SnO₂ followed by acetaldehyde steam reforming over Co(Fe)/ZnO catalyst and water gas shift reaction. Kinetic data have been obtained under different experimental conditions and a dynamic model has been developed for a tubular reformer loaded with catalytic monoliths for the production of the hydrogen required to feed a 1 kW PEMFC.     

Microchannel reactor modeling for combustion driven reforming of iso-octane

Coupling of exothermic and endothermic reactions in parallel microchannels is investigated through parametric variation of geometric and material properties in the context of hydrogen production by steam reforming of iso-octane, the surrogate for gasoline. Heat required for the endothermic reforming reaction is provided by the catalytic combustion of methane, the model compound for natural gas. The combination of steam reforming and combustion is modeled for a microchannel reactor configuration in which reactions and heat transfer take place in parallel, micro-sized, square-shaped flow paths with wall-coated catalysts. Thickness of the wall between microchannels, side-length of the microchannels and channel texture (straight-through vs. micro-baffled) are the geometric parameters being studied, while the use of different materials of construction - α-alumina, AISI-steel and iron - is also investigated. Instead of a fully-fledged 3-D mathematical model, the parametric runs are performed on a 2-D unit cell model which is justified to give results close to that of the former. When the wall thickness is increased from 1 × 10⁻⁴ to 4 × 10⁻⁴ m, the hydrogen yield, defined as moles of hydrogen produced per mole of iso-octane fed, increases by 42% because axial heat conduction in the wall becomes more pronounced and spreads the energy released by combustion into the reforming channel more effectively. Increase in the yield is more remarkable (110%) when the channel side-length is doubled from 2.8 × 10⁻⁴ to 5.6 × 10⁻⁴ m. Use of micro-baffles also enhances the hydrogen yield: in the 1-4 × 10⁻⁴ m wall thickness range, the average increase is 16.5%, which is attributed both to the enhancement in heat transfer coefficients and axial conduction in the wall. As for the material property parameter, due to having the highest thermal conductivity, hence the ability of conducting heat axially, iron serves as the best wall material on the basis of highest hydrogen yield, compared with alumina and steel.     

Hydrogen production using integrated methanol‐steam reforming reactor with various reformer designs for PEM fuel cells

A 95 mm × 40 mm × 15 mm compact reactor for hydrogen production from methanol‐steam reforming (MSR) is constructed by integrating a vaporizer, reformer, and combustor into a single unit. CuO/ZnO/Al₂O₃ is used as the catalyst for the MSR while the required heat is provided using Platinum (Pt) ‐catalytic methanol combustion. The reactor performance is measured using three reformer designs: patterned micro‐channel; inserted catalyst layer placed in a single plain channel; and catalyst coated directly on the bottom wall of single plain channel. Because of longer reactant residence time and more effective heat transfer, slightly higher methanol conversion can be obtained from the reformer with patterned microchannels. The experimental results show that there is no significant reactor performance difference in methanol conversion, hydrogen (H₂) production rate, and carbon monoxide (CO) composition among these three reformer designs. These results indicated that the flow and heat transfer may not play important roles in compact size reactors. The reformer design with inserted catalyst layer provides convenience in replacing the aged catalyst, which may be attractive in practical applications compared with the conventional packed bed and wall‐coated reformers. Copyright © 2011 John Wiley & Sons, Ltd.     

Experimental and computational investigations of a compact steam reformer for fuel oil and diesel fuel

The present work describes the optimisation of a compact steam reformer for light fuel oil and diesel fuel. The reformer is based upon a catalytically coated micro heat exchanger that thermally couples the reforming reaction with a catalytic combustion. Since the reforming process is sensitive to reaction temperatures and internal flow patterns, the reformer was modelled using a commercial CFD code in order to optimise its geometry. Fluid flow, heat transfer and chemical reactions were considered on both sides of the heat exchanger. The model was successfully validated with experimental data from reformer tests with 4 kW, 6 kW and 10 kW thermal inputs of light fuel oil. In further simulations the model was applied to investigate parallel flow, counter flow and cross flow conditions along with inlet geometry variations for the reformer. The experimental results show that the reformer design allows inlet temperatures below 773 K because of its internal superheating capability. The simulation results indicate that two parallel flow configurations provide fast superheating and high fuel conversion rates. The temperature increase inside the reactor is influenced by the inlet geometry on the combustion side.