Experimental assessment of hydrogen separation from H2/CH4 mixtures by the “steam-iron process” in an interconnected circulating fluidized bed reactor

An experimental study devoted to the continuous separation of hydrogen from its mixtures with methane in continuous regime is presented. The study was carried out in a single-vessel reactor divided into two interconnected adjoining chambers, operating as fluidized beds and with a continuous flow of solids between them. The process is based on iron oxide being selectively reduced or oxidized, depending on the redox nature of the gas stream being fed to the respective chamber (steam-iron process or SIP). Conditions to minimize gas leakage between the interconnected beds, avoiding insufficient circulation of solids, have been experimentally determined. The effects of different interconnecting designs, spatial gas velocities and partial pressures of reactants have been tested. Also the effects of several operating parameters (primarily hydrogen content in the inlet gas stream and spatial velocities) on process efficiency have been analyzed. Accordingly, a final configuration has been proposed for stable hydrogen separation and tested for several hours of time on stream.     

Hydrogen from ethanol by steam iron process in fixed bed reactor

This research is devoted to the use of ethanol (i.e. bio-ethanol) in the combined production and purification of hydrogen by redox processes. The process has been studied in a single lab scale fixed bed reactor. Iron oxides, apart from their remarked redox behavior, exert an important catalytic role allowing the complete decomposition of ethanol at temperatures in the range from 625 to 750 °C. The resulting gas stream (mainly H₂ and CO) reduces the solid to metallic iron. During a subsequent oxidation with steam, the solid can be regenerated to magnetite producing high purity hydrogen (suitable to be used in PEM fuel cells). Even though small amounts of coke are deposited during the reduction step, this is barely gasified by steam during the oxidation step (detection of CO_x in concentrations lower than 1 ppm). Influence of parameters like temperature, ethanol partial pressure and alternate cycles' effect has been studied in order to maximize the production of pure hydrogen.     

Characterization of Fe2O3/CeO2 oxygen carriers for chemical looping hydrogen generation

Fe₂O₃ is currently the most proper active metal oxide for chemical looping hydrogen generation (CLHG). However, supports are necessary to improve the reactivity and redox stability. CeO₂ can enhance the oxygen mobility, leading to high redox reactivity and carbon deposition resistance, which can be an excellent alternative support for oxygen carriers. In this paper, Fe₂O₃/CeO₂ oxygen carriers prepared by the co-precipitation method with different Fe₂O₃ loadings were investigated on a batch fluidized bed regarding the hydrogen yield and purity, redox reactivity and stability in CLHG with CO as fuel. The results showed that Fe6Ce4 is the best given comprehensive performance with no CO or CO₂ observed in the obtained hydrogen (detection limit 0.01% in volume). The oxygen mobility property for the reducible support CeO₂ and the physical contact between un-integrated Fe₂O₃ and CeO₂ could improve the reduction of Fe₂O₃. In addition, the formation of the hematite-like solid solution and perovskite-type CeFeO₃ could bring about abundant oxygen vacancies and promote the oxygen mobility, which contributes to the elimination of carbon deposition, counteracts the negative effect of serious sintering and guarantees the reactivity and redox stability of the Fe₂O₃/CeO₂ oxygen carriers. The Fe₂O₃/CeO₂ oxygen carriers were characterized by carbon monoxide temperature-programmed reduction measurement and X-ray diffraction patterns, and Fe6Ce4 was also selected to be characterized by scanning electron microscopy images and energy dispersive X-ray spectrometer analysis.     

Process integration for hydrogen production,purification and storage using iron oxides

Hydrogen production from natural gas using a Ni-based catalyst and its later hydrogen storage with some synthetic and natural iron oxides are presented. The Ni-based catalyst showed high methane conversion, close to the equilibrium one, when producing hydrogen from methane through catalytic partial oxidation (CPO) and wet-CPO with a low steam to carbon ratio (0.5). The solid solution formation observed in the Ni-based catalyst could have enhanced its stability. The iron oxides capacity for hydrogen storage was analysed with reduction–oxidation cycles at 973 K and atmospheric pressure. The natural oxides presented structural modifications, mainly due to sinterization, which negatively affected their storage capacity and stability.     

Finite element method simulations of heat flow in fixed bed solar water splitting redox reactors

An improved design for radiation absorption and heat flow into materials with low thermal conductivity is demonstrated. The design was developed for application in fixed bed two step solar water splitting redox reactors. The fixed bed was assumed to be made from porous ceramic. The low thermal conductivity of the porous ceramic redox material is compensated for by changing the profile of the fixed bed. The profiling used was wedges cut into the material which allows concentrated solar radiation to be incident on a larger area of redox material than for a flat monolith design. The design is demonstrated to efficiently transfer heat to the bulk and greatly reduce re-radiation. For a wedge 9 cm in depth and 1.6 cm wide at the opening, heated with 500 kW m⁻² incident radiation for 300 s, approximately double the amount of radiation is absorbed. The effects of thermal conductivity, emissivity and scaling on the efficiency of the design were investigated. The radiation absorption performance improved when scaled up. The improvement of the design over a flat plain bed is greater for lower emissivity. The improvement provided by the wedge design was found to decrease for increasing thermal conductivity, and eventually for high conductivity values it reduced performance. Using this method a larger amount of material with low thermal conductivity can be heated with the same power input and reduced radiation losses. The heat flow simulations were then coupled to an Arrhenius rate equation to investigate the possible improvement to the reaction efficiency and yield offered by the wedge design. Over a time of 300 s the efficiency and yield were seen to be approximately a factor of 4 times higher in the wedge case. Finally a concentrated solar cavity reactor based on the design is proposed.     

Hydrogen production through methane–steam cyclic redox processes with iron-based metal oxides

The redox performance of pure iron oxide (Fe₂O₃) and iron oxide modified with ceria (CeO₂) and/or zirconia (ZrO₂) as an oxygen carrier was investigated for hydrogen (H₂) production through a methane-steam redox process. The addition of both CeO₂ and ZrO₂ were found to be a more effective modification of Fe₂O₃ than the addition CeO₂ or ZrO₂ alone. It was found that the reducibility of Fe₂O₃ was enhanced by CeO₂ and the thermal stability of Fe₂O₃ was improved by ZrO₂. These results, therefore, led to the conclusion of the synergistic effect in the Fe₂O₃-CeO₂-ZrO₂ mixed oxide. As a result, both the redox activity and the thermal stability were significantly improved, and increases in H₂ yield and purity could be maintained by the modification. The redox temperature was found to have a significant effect on redox performance. The production of H₂ was considerably improved when the redox temperature was increased from 650 to 750 °C. The ZrO₂ concentration in Fe₂O₃-CeO₂-ZrO₂ mixed oxide samples was also found to influence performance with the highest H₂ yield observed at a ZrO₂ concentration of 75 wt.%. Although all materials tested showed a reduction in surface area in the first redox cycle, the change in surface area in subsequent cycles was found to be smaller and the yield of H₂ could be maintained at a constant level over a longer period for the mixed oxide containing 75 wt.% ZrO₂.     

Self-oriented iron oxide nanorod array thin film for photoelectrochemical hydrogen production

In this work, iron films were deposited on fluorine-tin-oxide coated glass substrate using radio frequency sputtering. Self-oriented iron oxide nanorod array thin films were obtained by anodizing the sputtered films. Anodization was carried out in an ethylene glycol solution containing 0.1 M NH₄F and various content of water. We studied the mechanism of anodization of iron thin films, and investigated the effects of some parameters on the properties of the iron oxide thin films.     

Solar water splitting for hydrogen production with monolithic reactors

The present work proposes the exploitation of solar energy for the dissociation of water and production of hydrogen via an integrated thermo-chemical reactor/receiver system. The basic idea is the use of multi-channelled honeycomb ceramic supports coated with active redox reagent powders, in a configuration similar to that encountered in automobile exhaust catalytic aftertreatment.Iron-oxide-based redox materials were synthesized, capable to operate under a complete redox cycle: they could take oxygen from water producing pure hydrogen at reasonably low temperatures (800 °C) and could be regenerated at temperatures below 1300 °C. Ceramic honeycombs capable of achieving temperatures in that range when heated by concentrated solar radiation were manufactured and incorporated in a dedicated solar receiver/reactor. The operating conditions of the solar reactor were optimised to achieve adjustable, uniform temperatures up to 1300 °C throughout the honeycomb, making thus feasible the operation of the complete cycle by a single solar energy converter.     

Stepwise production of syngas and hydrogen through methane reforming and water splitting by using a cerium oxide redox system

Stepwise production of syngas and hydrogen from ZrO₂-supported CeO₂ through methane reforming and water splitting was investigated in order to find proper operating conditions under which carbon deposition could be minimized. Recommendable operating temperature and time were 1073 K and 30 min for both the methane reforming and the water splitting. Even though the H₂/CO ratio during the methane reforming was maintained close to the desired ratio of 2, undesirable methane cracking occurred to a small extent and further reduction of Ce₂O₃ to metallic Ce by CH₄ and H₂ occurred to some extent. When the methane reforming-water splitting cyclic operations were repeated, the yields of syngas and hydrogen decreased considerably from the first cycle to the second cycle, but from the second cycle to the fifth cycle the gas yields were maintained nearly constant. As the CeO₂ content in the sample increased, the gas yields per mole of CeO₂ decreased but the gas productions per gram of sample increased.     

Two-step water splitting thermochemical cycle based on iron oxide redox pair for solar hydrogen production

This study deals with solar hydrogen production from the two-step iron oxide thermochemical cycle (Fe₃O₄/FeO). This cycle involves the endothermic solar-driven reduction of the metal oxide (magnetite) at high temperature followed by the exothermic steam hydrolysis of the reduced metal oxide (wustite) for hydrogen generation. Thermodynamic and experimental investigations have been performed to quantify the performances of this cycle for hydrogen production. High-temperature decomposition reaction (metal oxide reduction) was performed in a solar reactor set at the focus of a laboratory-scale solar furnace. The operating conditions for obtaining the complete reduction of magnetite into wustite were defined. An inert atmosphere is required to prevent re-oxidation of Fe(II) oxide during quenching. The water-splitting reaction with iron(II) oxide producing hydrogen was studied to determine the chemical kinetics, and the influence of temperature and particles size on the chemical conversion. A conversion of 83% was obtained for the hydrolysis reaction of non-stoichiometric solar wustite Fe_(1−y)O at 575 °C.     

A novel method for producing hydrogen from water with Fe enhanced by HS under mild hydrothermal conditions

In this paper, a novel method for producing hydrogen from water with Fe as a reductant promoted by HS⁻ under mild hydrothermal conditions was proposed. Results showed that hydrogen production significantly increased in the presence of HS⁻ compared to that in the absence of HS⁻. The obvious hydrogen production was achieved in a low reaction temperature of 250 °C and a very short reaction time (less than 2 h). The maximum yield of hydrogen production, which was defined as the percentage of produced hydrogen amount to theoretical one according to completive oxidation of Fe to Fe₃O₄ to produce hydrogen from water, was 34% at 300 °C. HS⁻ may act as a catalyst and a possible HS⁻-catalyzed mechanism was proposed. This process may provide a promising solution for biomass-driven hydrogen production from water combined with the process of reducing iron oxide into their zero-valent state by bio-driven chemicals, such as glycerin.     

Experimental investigation of chemical-looping hydrogen generation using Al2O3 or TiO2-supported iron oxides in a batch fluidized bed

Chemical-looping hydrogen generation (CLHG) is a novel technology for hydrogen production with inherent separation of CO₂. Three oxygen carriers Fe₂O₃ using inert materials Al₂O₃ or TiO₂ as support were prepared by mechanical-mixing method, i.e., Fe90Al10 (90%Fe₂O₃ + 10%Al₂O₃), Fe60Al40 (60%Fe₂O₃ + 40%Al₂O₃) and Fe60Ti40 (60%Fe₂O₃ + 40%Al₂O₃). Reactivity of the three oxygen carriers was first determined under CO reduction, steam oxidation and air oxidation atmospheres at 900 °C in a thermogravimetric analyzer. Then experiments to simulate the CLHG process were carried out in a batch fluidized bed. In the fluidized bed, all of the three oxygen carriers showed good reactivity over the multi-cycle experiments at 900 °C, and Fe60Al40 had the highest hydrogen yield. The reactivity of the oxygen carrier supported on Al₂O₃ was higher than that on TiO₂, which interacted with iron oxide forming FeTiO₃. The reactivity of Fe60Al40 was better than that of Fe90Al10. No deterioration of the oxygen carrier occurred after the multiple cycles, but for Fe90Al10 some agglomeration was detected. At 600-900 °C, higher temperature favored deeper reduction of iron oxide and increased the hydrogen production, while carbon deposition in the reduction period was suppressed with the rise of temperature. In the reduction, the conversion of fuel gas was constrained by thermodynamics in a single-stage reactor, and a compact fuel reactor was proposed for a full conversion of gaseous fuels.     

Improved iron oxide oxygen carriers for chemical looping hydrogen generation using colloidal crystal templated method

Iron oxide has been widely studied in chemical looping hydrogen generation (CLHG) process as an oxygen carrier, but fast decline of its activity in redox cycles due to sintering and agglomeration is one of the main drawbacks. In this work, the colloidal crystal templated (CCT) method was applied to synthesize Fe₂O₃/CeO₂ oxygen carrier and the mole ratio of Fe/Ce was 8:2, aiming to inhibit adjacent grains from agglomerating and improve the contact between the fuel gas and the oxygen carrier. The redox performances were evaluated with CO as fuel in a batch fixed bed reactor for 20 redox cycles, with oxygen carriers prepared by co-precipitation (CP) and sol-gel (SG) methods as references. X-ray diffraction (XRD), field emission scanning electron microscopy (SEM), and H₂-temperature programmed reduction (H₂-TPR) were used for characterization. The results showed that the calcination temperature lower than 750 °C was suitable for the CCT. The redox experiments showed that the H₂ yield and the redox stability for the oxygen carrier prepared by CCT were higher than those by co-precipitation and sol-gel methods. The H₂ yield of CCT oxygen carrier kept stable from the 3rd cycle and was 8.5 mmol/gOC in the 20th cycle. The pore structures resulting from CCT were different from another two oxygen carriers before and after the cycles, but maintained well through SEM images, leading to high activity and stability during redox cycles. The crystallite sizes of Fe₂O₃ and CeO₂ before and after redox cycles were the smallest for the CCT oxygen carrier from XRD patterns. In addition, H₂-TPR demonstrated that CCT oxygen carrier exhibited the highest reactivity.     

Molybdenum disulfide decorated palm oil waste activated carbon as an efficient catalyst for hydrogen generation by sodium borohydride hydrolysis

Development of cost-effective catalyst material with enhanced activity for hydrogen generation is highly desirable for hydrogen powered portable applications. In this work, molybdenum disulfide (MoS₂) incorporated on palm oil waste activated carbon (POAC) was used as a novel catalyst for enhanced hydrogen production by sodium borohydride (NaBH₄) hydrolysis. Hydrothermally synthesized MoS₂/POAC catalyst composite was characterized by SEM, EDX, XRD, FTIR, Raman, TGA and Surface area analysis. Characterization studies revealed the uniform and complete synthesis of MoS₂ nanoparticles on the POAC surface with crystallite size of 18.2 nm. The catalyst composite showed enhancement in thermal stability and reduction in specific surface area as compared with POAC. Hydrogen generation investigations showed ideal weight ratio of composite catalyst as 10:1 (w/w of POAC: MoS₂) and optimal catalyst to feed weight ratio as 0.07. MoS₂/POAC catalyst with 10 wt% of POAC loading recorded the maximum catalytic activity of 1170.66 mL/g min with lower activation energy of 39.1 kJ/mol. The catalyst composite exhibited virtuous reusability with a 28% loss in activity for nine cycle regeneration run. Thus, MoS₂/POAC catalyst system is highly attractive for commercial applicability and is a potential candidate for enhanced hydrogen production through NaBH₄ hydrolysis.     

Efficiency assessment of solar redox reforming in comparison to conventional reforming

Solar redox reforming is a process that uses solar radiation to drive the production of syngas from natural gas. This approach caught attention in recent years, because of substantially lower reduction temperatures compared to other redox cycles. However, a detailed and profound comparison to conventional solar reforming has yet to be performed. We investigate a two-step redox cycle with iron oxide and ceria as candidates for redox materials. Process simulations were performed to study both steam and dry methane reforming. Conventional solar reforming of methane without a redox cycle, i.e. on an established catalyst was used as reference. We found the highest efficiency of a redox cycle to be that of steam methane reforming with iron oxide. Here the solar-to-fuel efficiency is 43.5% at an oxidation temperature of 873 K, a reduction temperature of 1190 K, a pressure of 3 MPa and a solar heat flux of 1000 kW/m². In terms of efficiency, this process appears to be competitive with the reference process. In addition, production of high purity H₂ or CO is a benefit, which redox reforming has over the conventional approach.     

Enhanced hydrogen storage properties and mechanisms of magnesium hydride modified by transition metal dissolved magnesium oxides

Magnesium hydride (MgH₂) is a promising on-board hydrogen storage material due to its high capacity, low cost and abundant Mg resources. Nevertheless, the practical application of MgH₂ is hindered by its poor dehydrogenation ability and cycling stability. Herein, the influences and mechanisms of thin pristine magnesium oxide (MgO) and transition metals (TM) dissolved Mg(TM)O layers (TM = Ti, V, Nb, Fe, Co, Ni) on hydrogen desorption and reversible cycling properties of MgH₂ were investigated using first-principles calculations method. The results demonstrate that either thin pristine MgO or Mg(TM)O layer weakens the MgH bond strength, leading to the decreased structural stability and hydrogen desorption energy of MgH₂. Among them, the Mg(Nb)O layer exhibits the most pronounced destabilization effect on MgH₂. Moreover, the Mg(Nb)O layer presents a long-acting confinement effect on MgH₂ due to the stronger interfacial bonding strength of Mg(Nb)O/MgH₂ and the lower brittleness of Mg(Nb)O itself. Further analyses of electronic structures indicate that these thin oxide layers coating on MgH₂ surface reduce the bonding electron number of MgH₂, which essentially accounts for the weakened MgH bond strength and enhanced hydrogen desorption properties of modified MgH₂ systems. These findings provide a new avenue for enhancing the hydrogen desorption and reversible cycling properties of MgH₂ by designing and adding suitable MgO based oxides with high catalytic activity and low brittleness.     

Molybdenum nitride as a metallic photoelectrocatalyst for hydrogen evolution reaction via introduction of electron traps to improve the separation efficiency of photogenerated carriers

Metallic photoelectrocatalysts possess a wide light absorption range and the fast hydrogen evolution reaction (HER) kinetics, which can be used as the next generation of catalysts towards photoelectrocataytic HER. In this work, molybdenum nitride has been fabricated via an in-suit growth method on metal molybdenum substance (Mo₃N₂/Mo foil). The metallic and optical property of Mo₃N₂ was confirmed by the DFT calculations and experimental results from UV–visible absorption spectrum and valence X-ray photoelectron spectroscopy spectrum. Photocatalytic HER rate of Mo₃N₂ reached to 158.78 μmol h^?1 g^?1. Furthermore, Mo₃N₂–MoS₂/Mo foil was prepared to improve photoelectrocatalytic performance. Herein, a suitable energy band alignment for Mo₃N₂–MoS₂/Mo foil was proposed based on experiments and DFT calculations, and the formation of a heterojunction (Mo₃N₂–MoS₂) effectively suppressed the recombination of photo-generated carriers. The results of photoelectrocatalytic experiments suggested that the photocurrent density of Mo₃N₂–MoS₂/foil was effectively enhanced about 1.5 times than that of simplex Mo₃N₂/Mo foil. The electrochemical experiments (LSV and EIS) indicated that the metallic nature of Mo₃N₂ was also beneficial to electrocatalytic HER, and the overpotential of Mo₃N₂–MoS₂/Mo foil at 10 mA cm^?2 was ?173 mV. This work provides a potential candidate for photoelectrocatalytic electrodes.     

Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage

The development of a safe and efficient method for hydrogen storage is essential for the use of hydrogen with fuel cells for vehicular applications. Hollow glass microspheres (HGMs) have characteristics suitable for hydrogen storage and are expected to be a potential hydrogen carrier to be used for energy release applications. The HGMs with 10–100 μm diameters, 100–1000 Å pore width and 3–8 μm wall thicknesses are expected to be useful for hydrogen storage. In our research we have prepared HGMs from amber glass powder of particle size 63–75 μm using flame spheroidisation method. The HGMs samples with magnesium and iron loading were also prepared to improve the heat transfer property and thereby increase the hydrogen storage capacity of the product. The feed glass powder was impregnated with calculated amount of magnesium nitrate hexahydrate salt solution to get 0.2–3.0 wt% Mg loading on HGMs. Required amount of ferrous chloride tetrahydrate solution was mixed thoroughly with the glass feed powder to prepare 0.2–2 wt% Fe loaded HGMs. Characterizations of all the HGMs samples were done using FEG-SEM, ESEM and FTIR techniques. Adsorption of hydrogen on all the Fe and Mg loaded HGMs at 10 bar pressure was conducted at room temperature and at 200 °C, for 5 h. The hydrogen adsorption capacity of Fe loaded sample was about 0.56 and 0.21 weight percent for Fe loading 0.5 and 2.0 weight percentage respectively. The magnesium loaded samples showed an increase of hydrogen adsorption from 1.23 to 2.0 weight percentage when the magnesium loading percentage was increased from 0 to 2.0. When the magnesium loading on HGMs was increased beyond 2%, formation of nano-crystals of MgO and Mg was seen on the HGMs leading to pore closure and thereby reduction in hydrogen storage capacity.     

An iron ore-based catalyst for producing hydrogen and metallurgical carbon via catalytic methane pyrolysis for decarbonisation of the steel industry

Experiments to investigate the catalytic pyrolysis of methane using an iron ore-based catalyst were carried out to optimize catalytic activity and examine the purity of the carbon produced from the process for the first time. Ball milling of the iron ore at 300 rpm for varying times – from 30 to 330 min – was studied to determine the effect of milling time on methane conversion. Optimal milling for 270 min led to a five-fold increase in methane conversion from ca. 1%–5%. Further grinding resulted in a decline of methane conversion to 4% shown by SEM to correspond to an increase in particle size caused by agglomeration. Data from Raman and Mössbauer spectroscopy and H₂ temperature programmed reduction indicated a change in phase from magnetite to maghemite and hematite (at the particle surface) as the grinding time increased. Analysis of the carbon produced as a byproduct of the reaction indicated a highly pure material with the potential to be used as an additive for steel production.     

Absorption of hydrogen in tensile strained iron and high-carbon steel studied by electrochemical permeation and desorption techniques

Absorption of hydrogen in gradually tensile strained Armco iron and high-carbon steel, cathodically charged in 0.1 M NaOH solution, was studied using the electrochemical permeation and desorption techniques. Measurements of hydrogen permeation through specimens in the form of a membrane allowed determining the lattice diffusivity and concentration of hydrogen (diffusible hydrogen). The lattice diffusivity of hydrogen in iron (D = 6.2 × 10⁻⁵ cm²/s) was about 280 times higher than that in high-carbon steel (D = 2.2 × 10⁻⁷ cm²/s). In turn, a detailed analysis of the desorption rate of hydrogen from previously hydrogen charged and strained, cylindrical specimens made it possible to characterize hydrogen reversibly attached to traps. This trapped hydrogen made nearly a whole and a majority (from 70% to 85%, depending on strain) of the reversibly absorbed hydrogen in iron and high-carbon steel, respectively. In both studied materials, the amount of the trapped hydrogen strongly increased with strain. Moreover, in contrast to the diffusible hydrogen, evenly distributed in the charged specimen, the trapped hydrogen was mainly located within a subsurface region of the specimen. The estimated thickness of this subsurface region in iron was about 0.44 mm, whereas that in high-carbon steel was only about 0.017 mm. Consequently, the subsurface concentration of hydrogen in high-carbon steel was extremely high. It may be one of the reasons for more intensive hydrogen embrittlement of high-carbon (high-strength) steels in comparison with that of iron.