Pure hydrogen production by steam‐iron process: The synergic effect of MnO ₂ and Fe ₂ O ₃

Abstract

Summary In the energy transition from fossil to clean fuels, hydrogen plays a key role. Proton‐exchange membrane fuel cells (PEMFCs) represent the most promising hydrogen application, but they require a pure hydrogen stream (CO < 10 ppm). The steam iron process represents a technology for the production of pure H2, exploiting iron redox cycles. If renewable reducing agents are used, the process can be considered completely green. In this context, bio‐ethanol can be an interesting solution that is still not thoroughly explored. In this work, the use of ethanol as a reducing agent in the steam iron process will be investigated. Ethanol, at high temperature, decomposes mainly in syngas but can also form coke, which can compromise the process effectiveness, reacting with water and producing CO together with H2. In this work, the deposition of coke is avoided by controlling the duration of the reduction step; in fact, the data demonstrated that coke deposition is significantly dependent on reduction time. Tests were carried out in a fixed bed reactor using hematite (Fe2O3) as raw iron oxide adopting several reduction times (7 minutes‐25 minutes), which correspond to different amount of ethanol fed (5 mmolC2H5OH/gFe2O3‐17,95 mmolC2H5OH/gFe2O3). The effect of the addition of MnO2 to increase the reduction degree of iron oxides was explored using different amount of MnO2 (10 wt% and 40 wt% with respect to Fe2O3). The tests were performed at fixed temperatures of 675°C and atmospheric pressure. The optimization of the reduction time, in the chosen operating condition, performed only with Fe2O3, shows that, feeding an amount of 5 mmolC2H5OH/gFe2O3, coke deposition is avoided and, therefore, a pure H2 stream in oxidation is obtained. The addition of MnO2 leads to increased H2 yield and process efficiency, confirming its positive effect on the reduction degree of the solid bed. A reaction pathway to demonstrate the synergic effect of Fe2O3 and MnO2 in the reduction step was proposed in this article.

Efficient utilization of Al2O3 as structural promoter of Fe into 2 and 3 steps chemical looping hydrogen process: Pure H2 production from ethanol

“…At temperature higher than 500 °C ethanol starts to decompose producing mainly H2, CO and solid carbon. In an authors' previous work, the thermal ethanol decomposition pathway was studied at 675 °C and atmospheric pressure [22]. Based on the results, ethanol can be completely decomposed into a gaseous mixture mainly constituted by syngas and methane, which undergoes cracking reaction becoming responsible for coke formation.…”

“…In authors' previous work, pure H2 by 2 steps CLH is obtained using bioethanol as reducing agent and pure Fe2O3 as redox material. A relationship between the amount of ethanol fed in the reduction and the purity of H2 was demonstrated [22,23]. Specifically, if the complete reduction to Fe is avoided by monitoring the amount of ethanol fed, the lattice oxygen atoms of iron oxide are still able to convert solid carbon into CO and CO2, preventing its deposition and therefore guarantying a pure H2 stream in oxidation.…”

“…The fixed bed was constituted by 1 gram of Fe-based particles. The bench scale plant is described in details in the author's previous works [22,38].…”

Safe seasonal energy and hydrogen storage in a 1 : 10 single-household-sized pilot reactor based on the steam-iron process

“…The process lost relevance when H 2 could be produced more cheaply through steam reforming or water gas shi, 20 in the emerging petrochemical industry (1910 onwards). Recently, researchers' interest in this elegant process has been rekindled [21][22][23][24][25] due to the rise in the demand for CO-free H 2 as needed in fuel cells. Similarly, metal oxides enable efficient chemical looping combustion, which gained research interest due to the ease of CO 2 capture: [26][27][28][29][30] if a metal oxide is reduced using carbon-based fuel, the off-gas can be directly subjected to CO 2 capture without gas separation.…”

“…Among the broad variety of proposed metals, many excel in terms of reactivity if compared with Fe. 24,[31][32][33][34][35][36][37] Steinfeld et al, 16 however, recently provided an in-depth technical analysis conrming Fe as overall the most suitable metal for chemical looping H 2 storage and production if compared to Zn, Sn, Ge, W and Mo, in terms of reaction thermodynamics, H 2 storage density, resistance to sintering, safety and cost.…”