Efficient operation of autothermal microchannel reactors for the production of hydrogen by steam methane reforming

Efficient conversion of methane to hydrogen has emerged as a significant challenge to realizing fuel cell-based energy systems. Autothermal microchannel reactors, coupling of exothermic and endothermic reactions in parallel channels, have become one of the most promising technologies in the field of hydrogen production. Such reactors were utilized as an intensified design for conducting the endothermic steam methane reforming reaction. The energy required by the endothermic process is supplied directly through the separating plates of the reactor structure from the exothermic process occurring on the opposing side. Optimal design problems associated with transport phenomena in such an autothermal system were analyzed. Various methods for designing and operating autothermal reactors employed in steam methane reforming were discussed. Computational fluid dynamics simulations were performed to identify the underlying principles of process intensification, and to delineate several design and operational features of the intensified reforming process. The results indicated that the autothermal reactor is preferable to be thermally conductive to ensure its structural integrity and maximum operating regime. However, the thermal properties of the reactor structure are not essential due to efficient heat transfer existing between endothermic and exothermic process streams. A reactor design which minimizes the mass transfer resistance is highly required, and the channel dimension is of critical importance. Furthermore, the challenges presented by the efficient operation of the autothermal system were identified, along with demonstrating the implementation of transport management in order to improve overall reactor performance and to mitigate extreme temperature excursions.     

Characteristics of intraphase transport processes in methanol reforming microchannel reactors: A computational fluid dynamics study

Ignoring possible effects due to intraphase diffusion within catalyst layers is a common feature of computational fluid dynamics models developed for reforming microchannel reactors. Resistance to diffusion within the catalyst layers applied to such a reactor is often ignored on the grounds that the catalyst layers are sufficiently thin to allow reactants unrestricted access to all available reaction sites. However, this assumption is not necessarily correct, and intraphase diffusion effects could be important. Three-dimensional numerical simulations were carried out using computational fluid dynamics to investigate the characteristics of intraphase transport processes within the catalyst layers arranged in a thermally integrated methanol reforming microchannel reactor. The heat and mass transfer effects involved in the reforming process were evaluated, and the optimum thickness of catalyst layers was determined for the reactor. Particular focus was placed on how to optimize the thickness of catalyst layers in order to operate the reactor more efficiently. The results indicated that the performance of the reactor can be greatly improved by means of proper design of catalyst layer thickness to enhance heat and mass transfer into the catalyst layers. The thickness of the catalyst layers can be optimized to minimize diffusional resistance while maximizing methanol conversion and hydrogen yield. Thick catalyst layers offer higher reactor performance, whereas thin catalyst layers improve catalyst utilization and thermal uniformity. The thickness scale at which intraphase diffusion effects become noticeable was finally determined on the basis of reactor performance. The critical thickness was found to be about 0.10 mm, and catalyst layers should be designed beyond this dimension to achieve the desired level of conversion. The critical thickness will vary depending upon layer properties and operating conditions.     

Effect of catalytic washcoat shape and properties on mass transfer characteristics of microstructured steam-methanol reformers for hydrogen production

Many attempts have been made to improve mass transfer by reducing the size of reactors. However, such reduction will fairly quickly reach practical limitations and numerous difficulties still remain. Catalytic washcoat shape and properties may be critical design factors, but the mechanisms for their effects on mass transfer characteristics are still not fully understood. To effectively eliminate problems associated with mass transport phenomena in microstructured steam-methanol reformers, the effects of washcoat shape and properties were investigated in various situations by performing computational fluid dynamics simulations. The dependence of the solution on mass transfer characteristics was reduced to a small number of dimensionless parameters. A dimensionless mass transfer analysis was carried out in terms of the Sherwood, Schmidt, and pore Reynolds numbers. The results indicated that the rate of mass transfer is predominantly controlled by washcoat properties, and porosity and effective thermal conductivity are fundamentally important. The rate of the reforming reaction is typically controlled by kinetics at a temperature of 480 K and limited by mass transfer at a temperature of 580 K. The shape of washcoats affects the overall mass transfer characteristics, depending on the structural and thermal properties of washcoats. The shape effect is limited by heat transfer. A three-fold increase in effectiveness factor can be achieved by increasing the effective thermal conductivity of the washcoat. Design recommendations were finally made to improve transport characteristics for the systems.     

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

This paper addresses the issues related to the design and operation of steam reforming combined with catalytic combustion in thermally integrated microchannel reactors for hydrogen production. Comparisons were made between methanol and methane steam reforming, representing a low and a high temperature process respectively, under the same operating conditions to determine whether methanol-based thermally integrated systems can be more energy-efficient than methane-based ones. Computational fluid dynamics simulations were performed to gain insight into the reactor performance and thermal behavior. The effect of various design parameters was investigated to identify suitable ranges of operating conditions, and an analysis of heat and mass transfer was performed to design a highly efficient system. It was shown that steam reforming of both fuels is feasible in millisecond reactors under a variety of conditions, but very careful design is necessary. Methanol reforming can be more efficient, offering a better solution not only to simplify design but also to improve power and efficiency. The wall thermal conductivity is essential to the design and optimization of these systems, as it can significantly affect the overall energy balance. There is no significant difference in reactor performance between different channel heights at the same flow rate. The ratio of the flow rates on opposite sides of the reactor is an important design parameter and must be carefully adjusted to improve efficiency and eliminate hot spots. Finally, a simple operating strategy was proposed to achieve variable power output, and design recommendations were made.     

Millisecond methane steam reforming for hydrogen production: A computational fluid dynamics study

The potential of methane steam reforming to produce hydrogen in thermally integrated micro-chemical systems at short contact times was theoretically explored. Methane steam reforming coupled with methane catalytic combustion in microchannel reactors for hydrogen production was studied numerically. A two-dimensional computational fluid dynamics model with detailed chemistry and transport was developed. To provide guidelines for optimal design, reactor behavior was studied, and the effect of design parameters such as catalyst loading, channel height, and flow arrangement was evaluated. To understand how steam reforming can happen at millisecond contact times, the relevant process time scales were analyzed, and a heat and mass transfer analysis was performed. The importance of energy management was also discussed in order to obtain a better understanding of the mechanism responsible for efficient heat exchange between highly exothermic and endothermic reactions. The results demonstrated the feasibility of the design of millisecond reforming systems, but only under certain conditions. To achieve this goal, process intensification through miniaturization and the improvement in catalyst performance is very important, but not sufficient; very careful design and implementation of the system is also necessary to enable high thermal integration. The channel height plays an important role in determining the efficiency of heat exchange. A proper balance of the flow rates of the combustible and reforming streams is an important design criterion. Reactor performance is significantly affected by flow arrangement, and co-current operation is recommended to achieve a good energy balance within the system. The catalyst loading must be carefully designed to avoid insufficient reactant conversion or hot spots. Finally, operating windows were identified, and engineering maps for designing devices with desired power were constructed.     

Thermal management of methanol reforming reactors for the portable production of hydrogen

Three-dimensional numerical simulations were performed to address the thermal management issues associated with the design of a methanol reforming microchannel reactor for the portable production of hydrogen. The design of the reactor was fundamentally related to the direct coupling of reforming and combustion reactions by performing them on opposite sides of dividing walls in a parallel flow configuration. Effective autothermal operation was achieved through a combination of microchannel reactor technology with heat exchange in a direction perpendicular to the reacting fluid flow. Computational fluid dynamics simulations and thermodynamic analysis were carried out to investigate the effect of various design parameters on the characteristics of the generation, consumption, and exchange of thermal energy within the system. The results indicated that the ability to control temperature and temperature uniformity is of great importance to the performance of the system. The degree of temperature uniformity favorably affects the autothermal operation of the reactor. Temperature uniformity of the reactor can be improved by controlling the rate of heat transfer through a variety of factors such as wall thermal conductivity, fluid velocities, and dimensions. High wall thermal conductivity would be greatly beneficial to the performance of the system and the temperature uniformity of the reactor.     

Numerical studies on heat coupling and configuration optimization in an industrial hydrogen production reformer

Steam methane reforming furnaces are the most important devices in the hydrogen production industry. The highly endothermic reaction system requires reaction tubes in the furnace to have a large heat transfer area and to be operated under high temperature and pressure conditions. In order to enhance heat transfer efficiency and protect reaction tubes, the controlling and optimization of the furnace structure have increasingly received more and more research attention. As known from the furnace structure, it is essential to couple the exothermic combustion with the endothermic reforming reactions due to the highly interactive nature of the two processes. Thus, in this paper, the combustion process in the furnace was numerically studied by using computational fluid dynamics (CFD) to model the combustion chamber, coupled with methane steam reforming reaction inside the reaction tubes, defined by a plug flow model. A set of combustion models were compared for the furnace chamber and a plug flow reaction model was employed for reforming reaction tubes, and then a heat coupling process was established. The predicted flue gas temperature distribution showed that the heat transfer in the furnace was not uniform, resulting in hot spots and heat losses on the tube wall. Therefore, structure optimization schemes were proposed. Optimization on arrangements of the tubes and the nozzles promoted the uniform distribution of flue-gas temperature and then improved heat transfer efficiency, thereby enhancing performance of the steam reforming process.     

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.     

Experiments on hydrogen production from methanol steam reforming in the microchannel reactor

The entire experiments were conducted for microchannel methanol steam reforming, by which, the selection of catalyst, the operating parameters and the configuration of microchannels were discussed thoroughly. It was found that the higher the Cu concentration is, the more the corresponding active surface area of Cu will be, thereby improving the catalytic activity. The Cu-to-Zn ratio in Cu/ZnO/Al₂O₃ catalyst should be set at 1:1. The impacts of reaction temperature, feed flow rate, mixture temperature, and H₂O-to-CH₃OH molar ratio on the methanol conversion rate were also revealed and discussed. Characteristics of micro-reactors with various microchannels, including that 20 mm and 50 mm in length, as well as non-parallel microchannels, were investigated. It was found that the increase of microchannel length can improve the methanol conversion rate significantly. Besides, non-parallel microchannels help to maintain flow and temperature distribution uniformity, which can improve the performance of micro-reactor. In the present experiments, the presence of CO was under the condition that the methanol conversion rate was above 70%.     

A review on reforming bio-ethanol for hydrogen production

Bio-ethanol is a prosperous renewable energy carrier mainly produced from biomass fermentation. Reforming of bio-ethanol provides a promising method for hydrogen production from renewable resources. Besides operating conditions, the use of catalysts plays a crucial role in hydrogen production through ethanol reforming. Rh and Ni are so far the best and the most commonly used catalysts for ethanol steam reforming towards hydrogen production. The selection of proper support for catalyst and the methods of catalyst preparation significantly affect the activity of catalysts. In terms of hydrogen production and long-term stability, MgO, ZnO, _CeO2${\mathrm{CeO}}_2$, and _La2O3${\mathrm{La}}_2{\mathrm{O}}_3$ are suitable supports for Rh and Ni due to their basic characteristics, which favor ethanol dehydrogenation but inhibit dehydration. As Rh and Ni are inactive for water gas shift reaction (WGSR), the development of bimetallic catalysts, alloy catalysts, and double-bed reactors is promising to enhance hydrogen production and long-term catalyst stability. Autothermal reforming of bio-ethanol has the advantages of lesser external heat input and long-term stability. Its overall efficiency needs to be further enhanced, as part of the ethanol feedstock is used to provide low-grade thermal energy. Development of millisecond-contact time reactor provides a low-cost and effective way to reform bio-ethanol and hydrocarbons for fuel upgrading. Despite its early R&D stage, bio-ethanol reforming for hydrogen production shows promises for its future fuel cell applications.     

Simulation of steam reforming of biogas in an industrial reformer for hydrogen production

The paper aims to investigate the steam reforming of biogas in an industrial-scale reformer for hydrogen production. A non-isothermal one dimensional reactor model has been constituted by using mass, momentum and energy balances. The model equations have been solved using MATLAB software. The developed model has been validated with the available modeling studies on industrial steam reforming of methane as well as with the those on lab-scale steam reforming of biogas. It demonstrates excellent agreement with them. Effect of change in biogas compositions on the performance of industrial steam reformer has been investigated in terms of methane conversion, yields of hydrogen and carbon monoxide, product gas compositions, reactor temperature and total pressure. For this, compositions of biogas (CH₄/CO₂ = 40/60 to 80/20), S/C ratio, reformer feed temperature and heat flux have been varied. Preferable feed conditions to the reformer are total molar feed rate of 21 kmol/h, steam to methane ratio of 4.0, temperature of 973 K and pressure of 25 bar. Under these conditions, industrial reformer fed with biogas, provides methane conversion (93.08–85.65%) and hydrogen yield (1.02–2.28), that are close to thermodynamic equilibrium condition.     

Photocatalytic membrane reactors for hydrogen production from water

Hydrogen is considered today a promising environmental friendly energy carrier for the next future, since it produces no air pollutants or greenhouse gases when it burns in air, and it possesses high energy capacity. In the last decades great attention has been devoted to hydrogen production from water splitting by photocatalysis. This technology appears very attractive thanks to the possibility to work under mild conditions producing no harmful by-products with the possibility to use renewable solar energy. Besides, it can be combined with the technology of membrane separations making the so-called photocatalytic membrane reactors (PMRs) where the chemical reaction, the recovery of the photocatalyst and the separation of products and/or intermediates simultaneously occur. In this work the basic principles of photocatalytic hydrogen generation from water splitting are reported, giving particular attention on the use of modified photocatalysts able to work under visible light irradiation. Several devices to achieve the photocatalytic hydrogen generation are presented focusing on the possibility to obtain pure hydrogen employing membrane systems and visible light irradiation. Although many efforts are still necessary to improve the performance of the process, membrane photoreactors seem to be promising for hydrogen production by overall water splitting in a cost-effective and environmentally sustainable way.     

Generation-IV reactors and nuclear hydrogen production

There are dozens of hydrogen production methods and techniques from many sources such as fossil fuels, renewable energy sources and nuclear energy in the literature. Thermo-chemical methods are more efficient at higher temperatures to produce large quantities of hydrogen. In this study, a comparative overview of Generation VI nuclear reactor types for major hydrogen production methods have been researched in the literature and suggestions have been carried out.This research work is addressing that both electric power cycle and hydrogen production based on nuclear technologies need to be developed. Generation IV nuclear reactors can provide hydrogen for a worldwide hydrogen economy. Both thermo-chemical and electrolysis (hybrid) processes in hydrogen production have a promising future, especially when integrated with Generation IV nuclear power plants. Efficient heat transfer is required for both high temperature thermodynamic cycles and the high temperature steam electrolysis. Hence, highly efficient heat exchanger designs are one of the key technologies for that purpose.     

Steam reforming of technical bioethanol for hydrogen production

Essentially all work on ethanol steam reforming so far has been carried out using simulated bioethanol feedstocks, which means pure ethanol mixed with water. However, technical bioethanol consists of a lot of different components including sugars, which cannot be easily vaporized and steam reformed. For ethanol steam reforming to be of practical interest, it is important to avoid the energy-intensive purification steps to fuel grade ethanol. Therefore, it is imperative to analyze how technical bioethanol, with the relevant impurities, reacts during the steam reforming process. We show how three different distillation fractions of technical 2nd generation bioethanol, produced in a pilot plant, influence the performance of nickel- and ruthenium-based catalysts during steam reforming, and we discuss what is required to obtain high activity and long catalyst lifetime. We conclude that the use of technical bioethanol will result in a faster catalyst deactivation than what is observed when using pure ethanol–water mixtures because of contaminants remaining in the feed. However, the initial activity of the catalysts are not affected by this, hence it is important to not only focus on catalyst activity but rather on the lifetime of the catalyst.     

Optimal design of a sleeve-type steam methane reforming reactor for hydrogen production from natural gas

The three-dimensional computational fluid dynamics (CFD) model was used in a sleeve-type steam methane reforming (SMR) reactor for H₂ production of 2.5 Nm³/h from natural gas. The feed and combustion gases acted as a counter-current heat exchange owing to a narrow sleeve equipped between the combustor and catalyst-bed. The CFD results were validated against the experimental data of the SMR reactor with a sleeve gap size of 3 mm. The effect of the sleeve gap size and the flame shape on process performances such as H₂ production rate, thermal efficiency, and uniformity of catalyst-bed temperature was investigated using the CFD model. The sleeve gap size influenced the gas velocity inside the sleeve gap and the convective heat transfer. The SMR reactor with a sleeve gap size of 7 mm showed the highest H₂ production rate and thermal efficiency when comparing six sleeve gap sizes ranging from 2 to 10 mm. A new flame shape for the SMR reactor with the sleeve gap size of 7 mm was proposed to improve the process performances.     

Multi-objective optimization of hydrogen production process and steam reforming reactor design

This study proposes a multi-objective optimization approach to design a steam methane reforming (SMR) reactor and maximize the efficiency of the hydrogen production process. Out of 1782 possible variable combinations, only 50 iterations were performed, identifying three Pareto optimal that resulted in reactor size reductions of 50.53%, 35.56%, and 20.69%, respectively, compared to the reference reactor. The process efficiency for each optimal design varied slightly, with one achieving a 105.05% increase in efficiency, another remaining stable at 100.48%, and a third experiencing a slight decrease to 86.66% compared to the reference case. The results offer practical insights for planning an on-site distributed hydrogen production system, demonstrating that an increase in overall process efficiency can be achieved even with a reduced reactor size. This work is the first attempt to optimize a hydrogen production system by simultaneously considering overall process efficiency and SMR reactor design.     

Modelling of hydrogen perm-selective membrane reactors for catalytic methane steam reforming

Methane steam reforming is one of the most important pathways for producing high purity hydrogen. In this context, the use of fixed-bed catalytic reactors equipped with hydrogen perm-selective membranes is an interesting alternative for producing high purity hydrogen in one single step. In this work, this reactor is studied by means of numerical simulations using a 2D model, consisting of mass, energy and momentum balances. The fixed-bed is considered to be formed by Ru/SiO₂ catalyst particles, especially tailored for steam reforming at low temperature and steam-to-carbon ratio, whereas a composite palladium membrane was considered for hydrogen permeation. The model was validated with experimental data, and the adequacy of a simplified 1D model to simulate the membrane reactor was evaluated and discussed in comparison to the 2D model. Then, the model was used to study the influence of the main operating variables (inlet temperature, pressure, space velocity, steam excess and sweep gas rate in the permeate side) on the reactor performance. Finally, the optimum operating conditions, corresponding to a maximum hydrogen permeation rate, were determined, and the behaviour of the optimized reactor is analysed in detail.     

Dry reforming of methane has no future for hydrogen production: Comparison with steam reforming at high pressure in standard and membrane reactors

The methane dry-reforming and steam reforming reactions were studied as a function of pressure (1–20 atm) at 973 K in conventional packed-bed reactors and a membrane reactors. For the dry-reforming reaction in a conventional reactor the production yield of hydrogen rose and then decreased with increasing pressure as a result of the reverse water-gas shift reaction in which the hydrogen reacted with the reactant CO₂ to produce water. For the steam reforming reaction the production yield of hydrogen kept increasing with pressure because the forward water-gas shift reaction produced additional hydrogen by the reaction of CO with water. In the membrane reactors the methane conversion and the hydrogen production yields were higher for both the dry-reforming and steam reforming reactions, but for the dry reforming at high pressure half of the hydrogen was transformed into water. Thus, the dry-reforming reaction is not practical for producing hydrogen.     

Challenges for renewable hydrogen production from biomass

The increasing demand for H₂ for heavy oil upgrading, desulfurization and upgrading of conventional petroleum, and for production of ammonium, in addition to the projected demand for H₂ as a transportation fuel and portable power, will require H₂ production on a massive scale. Increased production of H₂ by current technologies will consume greater amounts of conventional hydrocarbons (primarily natural gas), which in turn will generate greater greenhouse gas emissions. Production of H₂ from renewable sources derived from agricultural or other waste streams offers the possibility to contribute to the production capacity with lower or no net greenhouse gas emissions (without carbon sequestration technologies), increasing the flexibility and improving the economics of distributed and semi-centralized reforming. Electrolysis, thermocatalytic, and biological production can be easily adapted to on-site decentralized production of H₂, circumventing the need to establish a large and costly distribution infrastructure. Each of these H₂ production technologies, however, faces technical challenges, including conversion efficiencies, feedstock type, and the need to safely integrate H₂ production systems with H₂ purification and storage technologies.