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.     

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.     

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.     

Exergy analysis of hydrogen production via steam methane reforming

The performance of hydrogen production via steam methane reforming (SMR) is evaluated using exergy analysis, with emphasis on exergy flows, destruction, waste, and efficiencies. A steam methane reformer model was developed using a chemical equilibrium model with detailed heat integration. A base-case system was evaluated using operating parameters from published literature. Reformer operating parameters were varied to illustrate their influence on system performance. The calculated thermal and exergy efficiencies of the base-case system are lower than those reported in literature. The majority of the exergy destruction occurs due to the high irreversibility of chemical reactions and heat transfer. A significant amount of exergy is wasted in the exhaust stream. The variation of reformer operating parameters illustrated an inverse relationship between hydrogen yield and the amount of methane required by the system. The results of this investigation demonstrate the utility of exergy analysis and provide guidance for where research and development in hydrogen production via SMR should be focused.     

Mass transport limitations in microchannel methanol-reforming reactors for hydrogen production

The potential of methanol reforming systems to greatly improve productivity in chemical reactors has been limited, due in part, to the effect of mass transfer limitations on the production of hydrogen. There is a need to determine whether or not a microchannel reforming reactor system is operated in a mass transfer-controlled regime, and provide the necessary criteria so that mass transfer limitations can be effectively eliminated in the reactor. Three-dimensional numerical simulations were carried out using computational fluid dynamics to investigate the essential characteristics of mass transport processes in a microchannel reforming reactor and to develop criteria for determining mass transfer limitations. The reactor was designed for thermochemically producing hydrogen from methanol by steam reforming. The mass transfer effects involved in the reforming process were evaluated, and the role of various design parameters was determined for the thermally integrated reactor. In order to simplify the mathematics of mass transport phenomena, use was made of dimensionless numbers or ratios of parameters that numerically describe the physical properties in the reactor without units. The results indicated that the performance of the reactor can be greatly improved by means of proper design of catalyst layer thickness and through adjusting feed composition to minimize or reduce mass transfer limitations in the reactor. There is not an effective method to reduce channel dimensions if the flow rate remains constant, or to reduce fluid velocities if the residence time is kept constant. The rate of the reforming reaction is limited by mass transfer near the entrance of the reactor and by kinetics further downstream, when the heat transfer in the autothermal system is efficient. Finally, the criteria that can be used to distinguish between different mass transport and kinetics regimes in the reactor with a first-order reforming reaction were presented.     

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.     

Sustainability assessment of hydrogen production by steam reforming

A specific methodology was applied to investigate the expected impact on sustainability of processes for hydrogen production by steam reforming of natural gas. Reference process schemes based on the current industrial state-of-the-art and on innovative steam reforming technologies were defined and assessed. The methodology allowed the calculation of a sustainability “fingerprint” of the reference schemes, based on sustainability key performance indicators. The results obtained evidenced the potential advantages of innovative processes, based on integrated conversion and internal membrane separation reactors. However, the valorization of the by-product streams is an essential requirement to reduce the impact on sustainability of these processes. Besides the improvement of the reaction section, the optimization of the overall separation efficiency and of energy requirement emerged as the key elements that control the overall sustainability performance of these processes.     

Assessment of CO2 capture options from various points in steam methane reforming for hydrogen production

Steam methane reforming (SMR) is currently the main hydrogen production process in industry, but it has high emissions of CO₂, at almost 7 kg CO₂/kg H₂ on average, and is responsible for about 3% of global industrial sector CO₂ emissions. Here, the results are reported of an investigation of the effect of steam-to-carbon ratio (S/C) on CO₂ capture criteria from various locations in the process, i.e. synthesis gas stream (location 1), pressure swing adsorber (PSA) tail gas (location 2), and furnace flue gases (location 3). The CO₂ capture criteria considered in this study are CO₂ partial pressure, CO₂ concentration, and CO₂ mass ratio compared to the final exhaust stream, which is furnace flue gases. The CO₂ capture number (N_cc) is proposed as measure of capture favourability, defined as the product of the three above capture criteria. A weighting of unity is used for each criterion. The best S/C ratio, in terms of providing better capture option, is determined. CO₂ removal from synthesis gas after the shift unit is found to be the best location for CO₂ capture due to its high partial pressure of CO₂. However, furnace flue gases, containing almost 50% of the CO₂ in produced in the process, are of great significance environmentally. Consequently, the effects of oxygen enrichment of the furnace feed are investigated, and it is found that this measure improves the CO₂ capture conditions for lower S/C ratios. Consequently, for an S/C ratio of 2.5, CO₂ capture from a flue gas stream is competitive with two other locations provided higher weighting factors are considered for the full presence of CO₂ in the flue gases stream. Considering carbon removal from flue gases, the ratio of hydrogen production rate and N_cc increases with rising reformer temperature.     

Novel structured catalysts configuration for intensification of steam reforming of methane

Structured catalysts, using highly conductive carriers, can improve the heat transfer along the catalytic bed, affording high performance with a flattened radial temperature gradient. The effect of thermal conductivity of structured carriers on highly endothermic Steam Reforming reaction is investigated. The performance of the structured catalysts, obtained on Cordierite and Silicon Carbide (SiC) monoliths, demonstrates the direct correlation between the thermal conductivity of the carrier, the methane conversion and the hydrogen productivity. The evaluation of the monolith configuration shows that the SiC “wall flow” guarantees a better axial and radial thermal distribution, with respect to the SiC “flow through”, resulting in better catalytic activity up to a temperature reaction of 750 °C. The comparison among the performance of the structured catalysts and the commercial 57-4MQ, provided by Katalco-JM, highlights the choice of structured catalysts, which require a lower temperature outside of the reactor, increasing the process efficiency.     

Numerical and experimental study on hydrogen production via dimethyl ether steam reforming

This work presents the characteristics of catalytic dimethyl ether (DME)/steam reforming based on a Cu–Zn/γ-Al₂O₃ catalyst for hydrogen production. A kinetic model for a reformer that operates at low temperature (200 °C–500 °C) is simulated using COMSOL 5.2 software. Experimental verification is performed to examine the critical parameters for the reforming process. During the experiment, superior Cu–Zn/γ-Al₂O₃catalysts are manufactured using the sol-gel method, and ceramic honeycombs coated with this catalyst (1.77 g on each honeycomb, five honeycombs in the reactor) are utilized as catalyst bed in the reformer to enhance performance. The steam, DME mass ratio is stabilized at 3:1 using a mass flow controller (MFC) and a generator. The hydrogen production rate can be significantly affected depending on the reactant's mass flow rate and temperature. And the maximum hydrogen yield can reach 90% at 400 °C. Maximum 8% error for the hydrogen yield is achieved between modeling and experimental results. These experiments can be further explored for directly feeding hydrogen to proton exchange membrane fuel cell (PEMFC) under the load variations.     

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.     

Exergoenvironmental analysis of a steam methane reforming process for hydrogen production

Steam methane reforming (SMR) is one of the most promising processes for hydrogen production. Several studies have demonstrated its advantages from the economic viewpoint. Nowadays process development is based on technical and economical aspects; however, in the near future, the environmental impact will play a significant role in the design of such processes. In this paper, an SMR process is studied from the viewpoint of overall environmental impact, using an exergoenvironmental analysis. This analysis presents the combination of exergy analysis and life cycle assessment. Components where chemical reactions occur are the most important plant components from the exergoenvironmental point of view, because, in general, there is a high environmental impact associated with these components. This is mainly caused by the exergy destruction within the components, and this in turn is mainly due to the chemical reactions. The obtained results show that the largest potential for reducing the overall environmental impact is associated with the combustion reactor, the steam reformer, the hydrogen separation unit and the major heat exchangers. The environmental impact in these components can mainly be reduced by improving their exergetic efficiency. A sensitivity analysis for some important exergoenvironmental variables is also presented in the paper.     

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.     

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.     

Controllability study of an ethanol steam reforming process for hydrogen production

A system for ethanol steam reforming and purification of carbon monoxide (CO) designed to feed a PEM fuel cell has been modelled. From the model, we study the sensitivity and controllability emphasizing the study of the influence of the temperature on the output variables of interest. The results of the study of controllability are used for the identification of the best control structures.     

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.     

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.     

Microwave-activated methanol steam reforming for hydrogen production

Methanol steam reforming (MSR) was carried out in a catalytic packed bed reactor under electrical and microwave heating using the two most common catalysts for this process-CuZnO/Al₂O₃ and PdZnO/Al₂O₃. Significant two-dimensional temperature gradients were found, especially in the latter case. Our results show that for the same average bed temperature, methanol conversion is higher under microwave heating (>10%). On the other hand, the product distribution is not affected by the heating mode. We demonstrate that even in cases where the maximum temperature along the entire height of the bed is significantly higher under electrical heating, conversion is either higher in the microwave case or approximately the same between the two heating modes. Finally, our experimental data indicate that a given methanol conversion can be achieved with lower net heat input to the reactor under microwave heating. This does not mean lower total energy consumption in the microwave process due to the limitations in the magnetron efficiency and the reflected power. However, it may be an implicit indication of higher temperature at metal sites than in bulk phase (microscale hot spot formation) due to the selective heating principle.     

Mechanistic study of bio-oil catalytic steam reforming for hydrogen production: Acetic acid decomposition

To clarify the understanding of the mechanism of bio-oil catalytic steam reforming, we selected acetic acid as a typical bio-oil model compound to study its detailed behavior in decomposition over an active stepped Ni surface by density functional theory calculations. The adsorption geometries and energies of various intermediates were reported. Linear correlations between the adsorption energy and the number of hydrogen atoms removed for CH_xCOOH, CH_xCOO, and CH_x species (x = 1–3) were found, with increments of ?1.56, ?0.81, and ?1.80 eV, respectively. Thirty-seven possible elementary reactions of acetic acid decomposition were proposed, and their activation energies, reaction energies, rate constants, and equilibrium constants were calculated. Acetic acid dissociation likely started via α-carbon dehydrogenation, OH dehydrogenation, and dehydroxylation. Combined with microkinetic modeling, the most preferable decomposition pathway was suggested as CH₃COOH → CH₃CO → CO + CH₃. The rate-determining step was CH₃COOH dehydroxylation to CH₃CO with an activation energy of 0.68 eV and a rate constant of 3.82 × 10⁸ s^?1. The formation of CH₃COO was dominant at high temperatures, whereas its decomposition occurred with difficulty.