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.     

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.     

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.     

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.     

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.     

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%.     

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.     

A performance study of methanol steam reforming in an A-type microchannel reactor

The methanol steam reforming (MSR) performance in a microchannel reactor is directly related to the flow pattern design of the microchannel reactor. Hydrogen production improvements can be achieved by optimal design of the flow pattern. In this study, an A-type microchannel reactor with a flow pattern design of one inlet and two outlets was applied to conduct the MSR for hydrogen production. The MSR performance of the A-type microchannel reactor was investigated through numerical analysis by establishing a three-dimensional simulation model and compared with that of the conventional Z-type microchannel reactor. Experiments were also conducted to test the MSR performance and validate the accuracy of the simulation model. The results showed that compared with the conventional Z-type microchannel reactor, the species distributions in the A-type microchannel reactor were more homogeneous. In addition, compared with the Z-type microchannel reactor, the A-type microchannel reactor was shown to effectively increase the methanol conversion rate by up to 8% and decrease the pressure drop by about 20%, regardless of a slightly higher CO mole fraction. It was also noted that with various quantities of microchannels and microchannel cross sections, the A-type microchannel reactor was still more competitive in terms of a higher methanol conversion rate and a lower pressure drop.     

Catalytic partial oxidation of methane for the production of syngas using microreaction technology: A computational fluid dynamics study

Catalytic partial oxidation of methane in high temperature environments under extremely short contact time conditions has emerged as a very promising reaction pathway for the production of syngas. This paper addresses the issues related to the favorable operating conditions for the process. Computational fluid dynamics simulations were performed to gain insight into the underlying mechanism and the key factors affecting primary reaction products. Particular emphasis was given to the role of homogenous and heterogeneous reaction pathways in determining the distribution of reaction products. The effect of preheating temperature, pressure, feed composition, and reactor dimension was investigated in order to identify conditions that will maximize the yield of syngas. Comparisons were made between air-feed and oxygen-feed systems. The relative importance of homogeneous and heterogeneous reactions was assessed, and the reaction pathways responsible for the production of syngas were identified. It was shown that there is a strong interplay between gas-phase and surface chemistry due to the competitive oxidation reactions occurring simultaneously in the system. The contribution of homogeneous and heterogeneous reaction pathways is highly dependent on the operating conditions. Gas-phase chemistry is favored at high preheating temperatures, high pressures, and large reactors, whereas surface chemistry is favored at low preheating temperatures, low pressures, and small reactors, with a tendency to shift towards higher syngas yields. It is particularly beneficial to utilize air instead of oxygen as the oxidant, especially at industrially relevant pressures, thereby inhibiting or avoiding the onset of undesired gas-phase chemistry.     

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.     

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.     

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.     

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.     

CFD simulation with detailed chemistry of steam reforming of methane for hydrogen production in an integrated micro-reactor

micro-reactor has drawn more and more attention in recent years due to the process intensification on basic transport phenomena in micro-channels, which would often lead to the improved reactor performance. Steam reforming of methane (SRM) in micro-reactor has great potential to realize a low-cost, compact process for hydrogen production via an evident shortening of reaction time from seconds to milliseconds. This work focuses on the detailed modeling and simulation of a micro-reactor design for SRM reaction with the integration of a micro-channel for Rh-catalyzed endothermic reaction, a micro-channel for Pt-catalyzed exothermic reaction and a wall in between with Rh or Pt-catalyst coated layer. The elementary reaction kinetics for SRM process is adopted in the CFD model, while the combustion channel is described by global reaction kinetics. The model predictions were quantitatively validated by the experimental data in the literature. For the extremely fast reactions in both channels, the simulations indicated the significance of the heat conduction ability of the reactor wall as well as the interplay between the exothermic and endothermic reactions (e.g., the flow rate ratio of fuel gas to reforming gas). The characteristic width of 0.5 mm is considered to be a suitable channel size to balance the trade-off between the heat transfer behavior in micro-channels and the easy fabrication of micro-channels.     

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.     

Thermodynamic analysis of methanol steam reforming to produce hydrogen for HT-PEMFC: An optimization study

A statistical modeling and optimization study on the thermodynamic equilibrium of methanol steam reforming (MSR) process was performed by using Aspen Plus and the response surface methodology (RSM). The impacts of operation parameters; temperature, pressure and steam-to-methanol ratio (H₂O/MeOH) on the product distribution were investigated. Equilibrium compositions of the H₂-rich stream and the favorable conditions within the operating range of interest (temperature: 25–600 °C, pressure: 1–3.0 atm, H₂O/MeOH: 0–7.0) were analyzed. Furthermore, ideal conditions were determined to maximize the methanol conversion, hydrogen production with high yield and to minimize the undesirable products such as CO, methane, and carbon. The optimum corresponding MSR thermodynamic process parameters which are temperature, pressure and H₂O/MeOH ratio for the production of HT-PEMFC grade hydrogen were identified to be 246 °C, 1 atm and 5.6, respectively.     

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.     

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.     

Hydrogen production by coupled catalytic partial oxidation and steam methane reforming at elevated pressure and temperature

Hydrogen production by coupled catalytic partial oxidation (CPO) and steam methane reforming of methane (OSMR) at industrial conditions (high temperatures and pressures) have been studied over supported 1 wt.% NiB catalysts. Mixture of air/CH₄/H₂O was applied as the feed. The effects of O₂:CH₄ ratio, H₂O:CH₄ ratio and the gas hourly space velocity (GHSV) on oxy-steam reforming (OSRM) were also studied. Results indicate that CH₄ conversion increases significantly with increasing O₂:CH₄ or H₂O:CH₄ ratio. However, the hydrogen mole fraction goes through a maximum, depending on reaction conditions, e.g., pressure, temperature and the feed gases ratios. Carbon deposition on the catalysts has been greatly decreased after steam addition. The supported 1 wt.% NiB catalysts exhibit high stability with 85% methane conversion at 15 bar and 800 °C during 70 h time-on-stream reaction (CH₄:O₂:H₂O:N₂ = 1:0.5:1:1.887). The thermal efficiency was increased from 35.8% by CPO (without steam) to 55.6%. The presented data would be useful references for further design of enlarged scale hydrogen production system.