Production and purification of hydrogen by biogas combined reforming and steam-iron process

Cobalt ferrite and hematite with minor additives have been tested for production and purification of high purity hydrogen from a synthetic biogas by steam-iron process (SIP) in a fixed bed reactor. A catalyst based in nickel aluminate has been included in the bed of solids to enhance the rate of the reaction of methane dry reforming (MDR). The reductants resulting from MDR are responsible for reducing the oxides based on iron that will, in the following stage, be oxidized by steam to release hydrogen with less than 50 ppm of CO. Coke minimization along reduction stages forces to operate such reactors above 700 °C for reductions, and as low as 500 °C for oxidations to avoid coke gasification. To avoid problems such as reactor clogging by coke in reductions and/or contamination of hydrogen by gasification of coke along oxidations, steam in small proportions has been included in the feed with the aim of minimizing or even avoiding formation of carbonaceous depositions along the reduction stage of SIP. Since steam is an oxidant, it exerts an inhibiting effect upon reduction of the oxide, that slows down the efficiency of the process. It has been proved that co-feeding low proportions of steam with an equimolar mixture of CH₄ and CO₂ (simulating a poor heating value desulphurized biogas) is able to avoid coke deposition, allowing the operation of both, reductions and oxidations, in isothermal regime (700 °C). Empirical results have been contrasted with data found in literature for similar processes based in MDR and combined (or mixed) reforming process (CMR), concluding that the combination of MDR + SIP proposed in this work, taking apart economic aspects and complex engineering, shows similar yields towards hydrogen, but with the advantage of not requiring a subsequent purification process.     

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.     

Separation and storage of hydrogen by steam-iron process: Effect of added metals upon hydrogen release and solid stability

During the last decade, the steam-iron process has re-emerged as a possible way to separate and/or storage pure hydrogen through the use of metallic oxides subjected to redox cycles. The most renamed candidate to achieve this goal has traditionally been iron oxide. Nevertheless, the study of its behaviour along repetitive reduction/oxidation stages has shown that the hydrogen storage density diminishes abruptly from the first cycle on.To cope with this problem, the inclusion of a second metal oxide in the solid structure has been tried. Isothermal experiments of reduction with hydrogen rich flows and oxidation with steam have been carried out with Al, Cr and Ce as second metals, in nominal amounts from 1% to 10 mol% added to the hematite structure, which has been synthesized in laboratory by coprecipitation. Series of up to seven cycles (reductions followed by oxidations in a thermogravimetric system acting as differential reactor for the gas) have shown that to that point, an almost repetitive behaviour can be obtained, recovering the magnetite (Fe₃O₄) structure after each oxidation step.Since the second metal oxide does not intervene in the reduction/oxidation process, the optimum content of second metal for each species has been determined with the aim to keep the highest hydrogen storage density along cycles.     

Molybdenum addition to modified iron oxides for improving hydrogen separation in fixed bed by redox processes

A system to produce hydrogen with high purity and without CO₂ emissions briefly consists of two operating units: production and separation. The coupling of the steam-iron process to the cracking of methane can manage that goal. However the steam-iron process needs an active and stable redox solid at moderate temperatures. The most suitable (pure iron oxide) suffers quick and strong deactivation mainly due to the structural changes upon reduction-oxidation cycles. Among those tested in our laboratory, one promising solid was proposed: 98 wt%Fe₂O₃–1.75 wt%Al₂O₃–0.25 wt%CeO₂. In this work, the expensive and rare cerium has been substituted by molybdenum. After optimizing the Mo amount in the solid, the long lasting experiments show that the new triple oxide, 98 wt%Fe₂O₃–1.75 wt%Al₂O₃–0.25 wt%MoO₃, in spite of some initial deactivation, maintains slightly better hydrogen production rates than the cerium sample. At temperature and conditions studied the Mo-solid was able to run, without coke formation, under real exhaust gas from natural gas thermocatalytic decomposition, producing about 8.1 g of high purity (>99.995%) hydrogen h⁻¹ kg of solid⁻¹. This means a natural gas processing of about 68 Nm³ h⁻¹ 1000 kg of solid⁻¹ (at 67% conversion of methane to hydrogen).     

Steam-iron process: Influence of steam on the kinetics of iron oxide reduction

The presence of low quantities of water vapour can seriously affect the kinetics of reduction of iron oxides when they are used as catalyst or to store and/or purify hydrogen from streams in the steam-iron process. Only 5% (v) of steam should be enough to inhibit the complete reduction of the solids. Since steam is a product of the reduction reaction, small amounts of water present in the reactive atmosphere can slow down the reduction itself. To account for the effect of the steam pressure during the reduction stage of the steam-iron process, two approaches have been considered and the resulting models, i.e. ‘competitive model’ and ‘inhibitive model’ have been tested against experimental measurements. Both models are based on the known Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory. The ‘competitive model’, accounts for the discretization of groups of moles of iron oxide/iron reducing and oxidizing with their own reaction rates. By using the kinetic parameters obtained from independent reduction and oxidation processes, this model is not capable of predicting properly the behaviour of the solid subjected to successive reductive and oxidative cycles. On the contrary, the ‘inhibitive model’, which takes into account the hydrogen and water vapour partial pressures in a Langmuir–Hinshelwood type kinetic constant dependency, seems to be very appropriate to predict correctly the effect of the presence of water in the reducing atmosphere.     

Hydrogen and electricity co-production plant integrating steam-iron process and chemical looping combustion

In this paper, steam-iron process (Fe looping) and NiO-based chemical looping combustion (Ni looping) are integrated for hydrogen production with inherent separation of CO₂. An integrated combined cycle based on the Fe and Ni loopings is proposed and modeled using Aspen Plus software. The simulation results show that at Fe-SR 815 °C, Fe-FR 815 °C, Ni-FR 900 °C and Ni-AR 1050 °C without supplementary firing, the co-production plant has a net power efficiency 14.12%, hydrogen efficiency 33.61% and an equivalent efficiency 57.95% without CO₂ emission. At a supplementary firing temperature of 1350 °C, the net power efficiency, hydrogen efficiency and the equivalent efficiency are 27.47%, 23.39% and 70.75%, respectively; the CO₂ emission is 365.36 g/kWh. The plant is attractive because of high-energy conversion efficiency and relatively low CO₂ emission; moreover, the hydrogen/electricity ratio can be varied in response to demand. The influences of iron oxide recycle rate, supplementary firing temperature, inert support addition and other parameters on the system performance are also investigated in the sensitive analyses.     

Steam-iron process kinetic model using integral data regression

A kinetic model describing the gas–solid non-catalytic reaction between iron oxides and hydrogen/methane gas mixtures has been proposed. This steam-iron process constitutes an interesting alternative in order to produce hydrogen without CO₂ generation, purifying streams of thermocatalytically decomposed natural gas. The study departed from a kinetic model obtained from differential regression of data acquired by thermogravimetry. This differential model (Avrami type) did not take into account some effects regarding the chemical equilibrium between reactants and products, neither those provided by the solid bed. To cope with this problem, some parameters were introduced in the kinetic model and experiments were performed in order to test the validity of the changes. These consisted of reduction steps with hydrogen and oxidations with steam along five alternated cycles in a fixed bed reactor. The refurbished reactor model (including kinetic model) consisted of a mono-dimensional fixed bed reactor working in non-stationary state. Initial parameter values were taken from the former kinetic model and later optimized with the aid of a Levenberg–Marquardt algorithm. The new model is able to predict with great accuracy the behaviour of the fixed bed reactor and represents an interesting tool for scale-up and process design.     

Pure hydrogen from pyrolysis oil using the steam-iron process

The novelty of using pyrolysis oil in the steam-iron process to produce pure hydrogen is introduced. In this process, products of pyrolysis oil gasification are used to reduce iron oxides which are subsequently oxidized with steam, resulting in pure hydrogen. Two process alternatives are considered: (i) a once-through concept in which cheap iron oxide (in our case sintered pellets of natural iron ore, Fe₂O₃) is used in one cycle, before further processing in a blast furnace, and (ii) a continuous system, in which specially developed iron oxides (in our case an ammonia catalyst) are cycled between a reduction and oxidation reactor. By injecting pyrolysis oil in a fluidized bed filled with Fe₃O₄ at 800 °C, it has been shown that CO and H₂ as well as coke by the gasification reactions contribute to the reduction. Experiments including a complete redox cycle with the ammonia catalyst have shown that a hydrogen production in the oxidation of 0.84 N m³/kg dry pyrolysis oil (LHV H₂/LHV oil = 0.4) can be obtained when the conversion of iron oxides are low (1.0%). The gas produced in the reduction step under these conditions contains 38% of the heating value of the input and has an LHV of 7.8 MJ/N m³ gas product. Deactivation of the iron oxides has been observed by a decreasing reduction rate in subsequent redox cycles. BET and SEM analysis showed a decrease in surface area, which could partly explain the observed deactivation.     

Thermodynamic analysis of a cyclic water gas-shift reactor (CWGSR) for hydrogen production

The cyclic water gas-shift reactor (CWGSR) is a cyclically operated fixed bed reactor for the removal of carbon monoxide from reformate gases. It is based on the repeated reduction of iron oxide by reformate gases and its subsequent oxidation by steam. To evaluate the thermodynamic limits of this reactor, we develop a model under the assumption of chemical equilibrium. For this purpose, we conduct a wave analysis which shows that the reactor behaviour is dominated by the movement of sharp reaction fronts. Depending on the positions of these fronts at cyclic steady state, five different operating regimes of the CWGSR can be identified. Besides the qualitative analysis of the regimes, the equilibrium model also offers a first quantitative analysis regarding the two performance parameters, i.e. fuel utilisation and product concentration. At 750 °C, a fuel utilisation of 55% can be achieved, and the molar hydrogen fraction in the product stream is up to 70%. The equilibrium model can be used for a first estimate of favourable design and operating parameters of the CWGSR.     

Cycle behaviour of iron ores in the steam-iron process

The use of several commercial iron ores usually employed as pigments, to store and supply pure hydrogen by means of the steam-iron process has been proposed and analyzed. The process roughly consists in repeated series of alternate reduction and oxidation steps in which a reducing stream (H₂ + CO, or in general H₂ enriched fuels) reacts with the iron oxide rendering the metal or a partially reduced oxide. Pure hydrogen is released during the re-oxidation with steam. The studied iron ores contain some impurities that accounting minor percentages (<10 wt%) enhance the behaviour of the solid. This improvement regards not only to the reduction and oxidation rate, but especially to the ability of the solid to maintain a given redox capacity along cycles. Also concerning this topic, the effect of the presence of these natural additives has been investigated in order to determine the inert behaviour of methane as a potential reducing agent. This study allowed the determination of the maximum temperature at which carbon formation is inhibited so that the subsequent released hydrogen will not be contaminated by carbon compounds.