Comparative study of activity of cerium oxide at thermal reduction temperatures of 1300–1550 °C for solar thermochemical two-step water-splitting cycle

Thermochemical two-step water-splitting using CeO₂ (cerium oxide) particles was studied to examine oxygen and hydrogen productivity and repeatability at thermal reduction (T-R) temperatures of 1300–1550 °C and water decomposition (W-D) temperatures of 400–1000 °C for the production of hydrogen from water using concentrated solar radiation as the energy source. The temperature dependency of oxygen and hydrogen productivity and the cyclic repeatability of CeO₂ are reported in this paper. The characteristic features of CeO₂ particles in the thermochemical two-step water-splitting cycle are compared with the well-known highly active reactive mediums of zirconia-supported Ni-ferrites (NiFe₂O₄/m-ZrO₂ and NiFe₂O₄/c-YSZ) and unsupported NiFe₂O₄.     

Thermochemical two-step water splitting cycles by monoclinic ZrO2-supported NiFe2O4 and Fe3O4 powders and ceramic foam devices

A thermochemical two-step water splitting cycle is examined for NiFe₂O₄ and Fe₃O₄ supported on monoclinic ZrO₂ (NiFe₂O₄/m-ZrO₂ and Fe₃O₄/m-ZrO₂) in order to produce hydrogen from water at a high-temperature. The evolution of oxygen and hydrogen by m-ZrO₂-supported ferrite powders was studied, and reproducible and stoichiometric oxygen/hydrogen productions were demonstrated through a repeatable two-step reaction. Subsequently, a ceramic foam device coated with NiFe₂O₄/m-ZrO₂ powder was made and examined as a water splitting device by the direct irradiation of concentrated Xe-light in order to simulate solar radiation. The reaction mechanism of the two-step water splitting cycle is associated with the redox transition of ferrite/wustite on the surface of m-ZrO₂. A hydrogen/oxygen ratio for these redox powder systems exhibited good reproducibility of approximately two throughout the repeated cycles. The foam device loaded NiFe₂O₄/m-ZrO₂ powder was also successful with respect to hydrogen production through 10 repeated cycles. A ferrite conversion of 24-76% was obtained over an irradiation period of 30 min.     

Ferrite/zirconia-coated foam device prepared by spin coating for solar demonstration of thermochemical water-splitting

A Ferrite/zirconia foam device in which reticulated ceramic foam was coated with zirconia-supported Fe₃O₄ or NiFe₂O₄ as a reactive material was prepared by a spin-coating method. The spin-coating method can shorten the preparation period and reduce the coating process as compared to the previous wash-coating method. The foam devices were examined for hydrogen productivity and cyclic reactivity in thermochemical two-step water-splitting. The reactivity of these foam devices were studied for the thermal reduction of ferrite on a laboratory scale using a sun simulator to simulate concentrated solar radiation, while the thermally reduced foam devices were reacted with steam in another quartz reactor under homogeneous heating in an infrared furnace. The most reactive foam device, NiFe₂O₄/m-ZrO₂/MPSZ, was tested for successive two-step water-splitting in a windowed single reactor using solar-simulated Xe-beam irradiation with a power input of 0.4-0.7 kW_th. The production of hydrogen continued successfully in the 20 cycles that were demonstrated using the NiFe₂O₄/m-ZrO₂/MPSZ foam device. The NiFe₂O₄/m-ZrO₂/MPSZ foam device produced hydrogen at a rate of 1.1-4.6 cm³ per gram of device through 20 cycles and reached a maximum ferrite conversion of 60%.     

Oxygen-releasing step of nickel ferrite based on Rietveld analysis for thermochemical two-step water-splitting

We investigated the thermal reduction (T-R) of NiFe₂O₄, either supported by m-ZrO₂ or unsupported, as the oxygen-releasing step of a solar thermochemical water splitting cycle based on a ferrite/wustite redox system, by performing the Rietveld analysis using powder X-ray diffraction. The solid materials obtained after the T-R step at 1300–1400 °C were subjected to Rietveld analysis. The amounts and chemical compositions of the wustite phase produced by the T-R step and the remaining ferrite phase were identified quantitatively. Chemical reaction formulas for the different T-R temperatures were determined from the results. Consistency for the chemical reactions of the thermal reduction was discussed and evaluated comparing the O₂ amounts predicted by the chemical reaction formulas and measured experimentally by mass spectrometry.     

CO2 gasification of coal cokes using internally circulating fluidized bed reactor by concentrated Xe-light irradiation for solar gasification

For the solar thermochemical gasification of coal coke to produce CO + H₂ synthetic gas using concentrated solar radiation, a windowed reactor prototype is tested and demonstrated at laboratory scale for CO₂ gasification of coal coke using concentrated Xe light from a 3-kW_th sun simulator. The reactor was designed to be combined with a solar reflective tower or beam-down optics. The results for gasification performance (CO production rate, carbon conversion, and light-to-chemical efficiency) are shown for various CO₂ flow rates and ratios. A kinetics analysis based on homogeneous and shrinking core models and the temperature distributions of the prototype particle bed are compared with those for a conventional fluidized bed reactor tested under the same Xe light irradiation and CO₂ flow-rate conditions. The effectiveness and potential impacts of internally circulating fluidized bed reactors for enhancing gasification performance levels and inducing consistently higher bed temperatures are discussed in this paper.     

Thermal conversion efficiency of producing hydrogen enriched syngas from biomass steam gasification

This paper presents the results from an experimental study on the energy conversion efficiency of producing hydrogen enriched syngas through uncatalyzed steam biomass gasification. Wood pellets were gasified using a 100 kW_th fluidized bed gasifier at temperatures up to 850 °C. The syngas hydrogen concentration and cold gas efficiency were found to increase with both bed temperature and steam to biomass weight ratio, reaching a maximum of 51% and 124% respectively. The overall energy conversion to syngas (based on heating value) also increased with bed temperature but was inversely proportional to the steam to biomass ratio. The maximum energy conversion to syngas was found to be 68%. The conversion of energy to hydrogen (by heating value) increased with gasifier temperature and gas residence time, but was found to be independent of the S/B ratio. The maximum conversion of all energy sources to hydrogen was found to be 25%.     

Experimental study on N2O emission in O2/CO2 combustion with high oxygen concentration in circulating fluidized bed

To characterize the N₂O formation and fuel nitrogen conversion in an oxy-fuel circulating fluidized bed (CFB) combustor with high oxygen concentration, tests were carried out by analysing the axial concentrations of N₂O in a 50 kW_th CFB combustor. The conversion ratios from fuel nitrogen to gaseous N-containing pollutants were calculated. The initial N₂O concentrations in the bottom of the combustor were similar between oxy-fuel firing and air-firing. The axial N₂O formation was more in oxy-fuel combustion than in air-firing, improving the N₂O emission during 50% O₂/50% CO₂ combustion. The atmospheric variation significantly affected the conversion ratio from fuel nitrogen to N₂O. In addition, the conversion from fuel nitrogen to N₂O was much higher than that to NO. As a result, the N₂O emission during oxy-fuel CFB combustion cannot be ignored. Gas staging little influenced the N₂O emission. With the increasing ratio of secondary gas, the initial N₂O formation in the dense zone increased, while the axial N₂O formation along the combustor declined. By analysing the conversion ratios of fuel nitrogen, it was also found that gas staging obviously affected the conversion ratio from fuel nitrogen to NO by enhancing the NO to N₂ conversion. However, gas staging did not impact the conversion ratio from fuel nitrogen to N₂O.     

Water splitting for hydrogen production with ferrites

The water splitting reaction by a thermo-chemical cycle using ferrites was investigated for H₂ production. In the first step (activation step), ferrites were thermally reduced at 1200 °C to form an oxygen-deficient ferrite. In the second step (water splitting step), the activated ferrites were oxidized by water at 800 °C to produce hydrogen. Among the prepared ferrites, Ni-ferrite was found to be the most suitable for H₂ production. NiFe₂O₄ produced an average of 0.442 cm³/g cycle of H₂. The H₂ productivity of the Ni-ferrite was much higher than that of the other ferrites at the same temperature. XRD showed that the crystal structure of NiFe₂O₄ during the redox reaction was not changed during the repeated cycles, indicating that NiFe₂O₄ was an excellent material in terms of structural stability and durability.     

Redox cycling of CuFe2O4 supported on ZrO2 and CeO2 for two-step methane reforming/water splitting

CuFe₂O₄ supported on ZrO₂ and CeO₂ for two-step methane reforming was evaluated to determine if it could enhance the reactivity, CO selectivity and thermal stability of CuFe₂O₄. Two-step methane reforming consists of a syngas production step and a water splitting step. CuFe₂O₄ supported on ZrO₂ and CeO₂ was prepared using an aerial oxidation method. Non-isothermal methane reduction was carried out on TGA to compare the reactivity of CuFe₂O₄/ZrO₂ and CuFe₂O₄/CeO₂. In addition, a syngas production step was performed at 900 °C and water splitting was conducted at 800 °C alternatively five times to compare the methane conversion, CO selectivity, cycle ability and hydrogen production by water splitting in a fixed bed reactor. If the 1st syngas production step results are excluded due to over-oxidation, CuFe₂O₄/ZrO₂ and CuFe₂O₄/CeO₂ showed approximately 74.0–82.8% and 60.3–87.5% methane conversion, respectively, and 44.0–47.8% and 65.2–81.5% CO selectivity, respectively. Using CeO₂ and ZrO₂ as supports effectively improved the reactivity and methane conversion compared to CuFe₂O₄. CuFe₂O₄/ZrO₂ showed high methane conversion due to the high phase stability and thermal stability of ZrO₂ but the selectivity was not improved. After 5 successive cycles, the CeFeO₃ phase was found on CuFe₂O₄/CeO₂. Furthermore, methane conversion, CO selectivity and the amounts of hydrogen production of CuFe₂O4/CeO₂ increased with increasing number of cycles. Additional test up to the 11th cycle on CuFe₂O₄/CeO₂ revealed that CeO₂ is a better support that ZnO₂ in terms of the reactivity and CO selectivity.     

Thermochemical hydrogen production by a redox system of ZrO2-supported Co(II)-ferrite

The thermochemical two-step water splitting was examined on ZrO₂-supported Co(II)-ferrites below 1400 °C, for purpose of converting solar high-temperature heat to clean hydrogen energy as storage and transport of solar energy. The ferrite on the ZrO₂-support was thermally decomposed to the reduced phase of wustite at 1400 °C under an inert atmosphere. The reduced phase was reoxidized with steam on the ZrO₂-support to generate hydrogen below 1000 °C in a separate step. The ZrO₂-supporting alleviated the high-temperature sintering of iron oxide. As the results, the ZrO₂-supported ferrite realized a greater reactivity and a better repeatability of the cyclic water splitting than the conventional unsupported ferrites. The Co_xFe_3−xO₄/ZrO₂ with the x value of around 0.4–0.7 was found to be the promising working material for the two-step water splitting when thermally reduced at 1400 °C under an inert atmosphere.     

Modeling of the chemical-looping combustion of methane using a Cu-based oxygen-carrier

A mathematical model for a bubbling fluidized bed has been developed to simulate the performance of the fuel-reactor in chemical-looping combustion (CLC) systems. This model considers both the fluid dynamic of the fluidized bed and freeboard and the kinetics of reduction of the oxygen-carrier, here CuO impregnated on alumina. The main outputs of the model are the conversion of the carrier and the gas composition at the reactor exit, the axial profiles of gas concentrations and the fluid dynamical structure of the reactor. The model was validated using measurements when burning CH₄ in a 10 kW_th prototype using a Cu-based oxygen-carrier. The influence of the circulation rate of solids, the load of fuel gas, the reactor temperature and size of the oxygen-carrier particles were analyzed. Combustion efficiencies predicted by the model showed a good agreement with measurements. Having validated the model, the implications for designing and optimizing a fuel-reactor were as follows. The inventory of solids for a high conversion of the fuel was sensitive to the reactor’s temperature, the solids’ circulation rate and the extent to which the solids entering to the reactor had been regenerated. The optimal ratio of oxygen-carrier to fuel was found to be 1.7–4 for the Cu-based oxygen-carrier used here. In this range, the inventory of solids to obtain a combustion efficiency of 99.9% at 1073 K was less than 130 kg/MW_th. In addition, the model’s results were very sensitive to the resistance to gas diffusing between the emulsion and bubble phases in the bed, to the decay of solids’ concentration in the freeboard and to the efficiency contact between gas and solids in the freeboard. Thus, a simplified model, ignoring any restriction to gas and solids contacting each other, will under-predict the inventory of solids by a factor of 2–10.     

Process simulation and techno-economic assessment of SER steam gasification for hydrogen production

In the SER (sorption enhanced reforming) gasification process a nitrogen-free, high calorific product gas can be produced. In addition, due to low gasification temperatures of 600–750 °C and the use of limestone as bed material, in-situ CO₂ capture is possible, leading to a hydrogen-rich and carbon-lean product gas. In this paper, results from a bubbling fluidised bed gasification model are compared to results of process demonstration tests in a 200 kW_th pilot plant.Based upon that, a concept for the hydrogen production via biomass SER gasification is studied in terms of efficiency and feasibility. Capital and operational expenditures as well as hydrogen production costs are calculated in a techno-economic assessment study. Furthermore, market framework conditions are discussed under which an economic hydrogen production via SER gasification is possible.     

Steam gasification of coal cokes in an internally circulating fluidized bed of thermal storage material for solar thermochemical processes

A laboratory-scale prototype windowed internally circulating fluidized-bed reactor made of quartz sand and coal coke particles was investigated for steam gasification using concentrated Xe-light radiation as the energy source. The quartz sand was used as a chemically inert bed material for the fluidized bed, while the coal coke particles functioned as the reacting particles for the endothermic gasification reaction. The advantages of using quartz sand as the bed material for the directly irradiated gasification reactor are as follows: (1) The bed height is maintained at a constant level during the gasification. (2) The quartz sand functions as a thermal transfer/storage medium inside the reactor. The gasification performances such as the production rates of CO, H₂, and CO₂; carbon conversion; and light-to-chemical energy conversion were evaluated for the fluidized-bed reactor with a thermal transfer/storage medium (quartz sand). The effects of using the bed material (quartz sand) on the gasification performance are described in this paper.     

Autothermal chemical looping reforming process of different fossil liquid fuels

The autothermal Chemical-Looping Reforming (a-CLR) is a process where syngas is produced with two main advantages; there are captured CO₂ emissions and the heat required for the syngas production is generated by the process itself. A Ni-based material is used as oxygen carrier circulating between two fluidized bed reactors: the fuel and air reactors. In this work, the auto-thermal conditions in a global H₂ production process, integrated by the a-CLR process and a Water Gas Shift reactor, using different liquid fossil fuels were theoretically determined. The hydrogen production per mol of carbon in the fuel was similar for all fossil fuels, taking a value of 2.2 at the optimal operating temperature (700 °C). In addition, the possibility of working at low temperature for a maximum H₂ production was experimentally demonstrated in a continuous 1 kW_th a-CLR unit.     

A rotating fluidized bed reactor for rapid temperature ramping in two-step thermochemical water splitting

Two-step thermochemical water splitting (TWS) is a promising carbon-free/low-carbon technology for producing hydrogen in a mass production scale, in which water is dissociated in the presence of metal oxide-based catalysts via redox cycles driven by thermal energy. While active research is underway to develop high-performance metal oxide catalysts, less attention has been paid to reactor design and relevant system analysis, which are essential for constructing an actual system. The gas and solid flow configuration is one of the key design parameters in reactor design that determines thermodynamic and kinetic characteristics of the entire two-step TWS system. In this study, we propose a rotating fluidized bed reactor design wherein the rotating current flow configuration allows a much larger relative velocity between sweep gas and redox particles compared to conventional flow configurations. The rotating current flow configuration significantly improves the temperature ramp rate of redox particles via enhanced heat transfer between particles and sweep gas. Through thermodynamic and kinetics analysis of the reactor system, we show that the large temperature ramp rate of the proposed reactor results in a considerable improvement in the hydrogen yield per hour. This work adds a new dimension to the reactor design for two-step TWS.     

The Sotacarbo coal gasification experimental plant for a CO2-free hydrogen production

Being hydrogen the most promising energy carrier for distributed power generation and being the coal-to-hydrogen technologies far from their large-scale application, mainly due to their high costs, Sotacarbo has recently built up a pilot platform for the combined production of hydrogen and electrical energy from coal. The platform includes two different units: a 5 MW_th demonstrative plant and a 200 kW_th pilot plant (feed with a maximum of 35 kg/h of coal).     

Hydrogen production via solar-aided water splitting thermochemical cycles: Combustion synthesis and preliminary evaluation of spinel redox-pair materials

Redox-pair-based thermochemical cycles are considered as a very promising option for the production of hydrogen via renewable energy sources like concentrated solar energy and raw materials like water. This work concerns the synthesis of various spinel materials of the iron and aluminum families via combustion reactions in the solid and in the liquid-phase and the testing of their suitability as redox-pair materials for hydrogen production by water splitting via thermochemical cycles. The effects of reactants' stoichiometry (fuel/oxidizer) on the combustion synthesis reaction characteristics and on the products' phase composition and properties were studied. By fine-tuning the synthesis parameters, a wide variety of single-phase, pure and well crystallized spinels could be controllably synthesized. Post-synthesis, high-temperature calcination studies under air and nitrogen at the temperature levels encountered during solar-aided thermochemical cyclic operation have eliminated several material families due to phase composition instabilities and identified among the various compositions synthesized NiFe₂O₄ and CoFe₂O₄ as the two most suitable for cyclic water splitting – thermal reduction operation. First such thermochemical cyclic tests between 800 and 1400 °C with NiFe₂O₄ and CoFe₂O₄ in powder form in a fixed bed laboratory reactor have demonstrated capability for cyclic operation and alternate hydrogen/oxygen production at the respective cycle steps for both materials. Under the particular testing conditions the two materials exhibited hydrogen/oxygen yields of the same magnitude and similar temperatures of oxygen release during thermal reduction.     

Reactivity deterioration of NiO/Al2O3 oxygen carrier for chemical looping combustion of coal in a 10 kWth reactor

A relatively long-term experiment for chemical looping combustion of coal with NiO/Al₂O₃ oxygen carrier was carried out in a 10 kW_th continuous reactor of interconnected fluidized beds, and 100 h of operation was reached with the same batch of the oxygen carrier. The reactivity deterioration of the oxygen carriers was present during the experimental period. The reactivity deterioration of reacted oxygen carriers at different experimental stages was evaluated using X-ray diffraction (XRD), scanning electron microscope (SEM), and X-ray fluorescence spectrometer. SEM analysis showed no significant change in the morphology of the nickel-based oxygen carrier at the fuel reactor temperature ?940 °C, but loss of surface area and porosity of reacted oxygen carriers was observed when the fuel reactor temperature exceeded 960 °C. The results show that the sintering effect have mainly contributed to the reactivity deterioration of reacted oxygen carriers in the CLC process for coal, while the effects of coal ash and sulfur can be ignored. The oxidization of reduced oxygen carrier with air was an intensive exothermic process, and the high temperature of oxygen carrier particles led to sintering on the surface of oxygen carrier particles in the air reactor. Attention must be paid to control the external circulation of oxygen carrier particles in the interconnected fluidized beds in order to efficiently transport heat from the air reactor to the fuel reactor, and reduce the temperature of oxygen carrier particles in the air reactor. Improvement of reactivity deterioration of reacted oxygen carriers was achieved by the supplement of steam into the fuel reactor. Nevertheless, NiO/Al₂O₃ is still one of the optimal oxygen carriers for chemical looping combustion of coal if the sintering of oxygen carrier is minimized at the suitable reactor temperature.     

Characterization of chemical looping combustion of coal in a 1 kWth reactor with a nickel-based oxygen carrier

Chemical looping combustion is a novel technology that can be used to meet the demand on energy production without CO₂ emission. To improve CO₂ capture efficiency in the process of chemical looping combustion of coal, a prototype configuration for chemical looping combustion of coal is made in this study. It comprises a fast fluidized bed as an air reactor, a cyclone, a spout-fluid bed as a fuel reactor and a loop-seal. The loop-seal connects the spout-fluid bed with the fast fluidized bed and is fluidized by steam to prevent the contamination of the flue gas between the two reactors. The performance of chemical looping combustion of coal is experimentally investigated with a NiO/Al₂O₃ oxygen carrier in a 1 kW_th prototype. The experimental results show that the configuration can minimize the amount of residual char entering into the air reactor from the fuel reactor with the external circulation of oxygen carrier particles giving up to 95% of CO₂ capture efficiency at a fuel reactor temperature of 985 °C. The effect of the fuel reactor temperature on the release of gaseous products of sulfur species in the air and fuel reactors is carried out. The fraction of gaseous sulfur product released in the fuel reactor increases with the fuel reactor temperature, whereas the one in the air reactor decreases correspondingly. The high fuel reactor temperature results in more SO₂ formation, and H₂S abatement in the fuel reactor. The increase of SO₂ in the fuel reactor accelerates the reaction of SO₂ with CO to form COS, and COS concentration in the fuel reactor exit gas increases with the fuel reactor temperature. The SO₂ in the air reactor exit gas is composed of the product of sulfur in residual char burnt with air and that of nickel sulfide oxidization with air in the air reactor. Due to the evident decrease of residual char in the fuel reactor with increasing fuel reactor temperature, it results in the decrease of residual char entering the air reactor from the fuel reactor, and the decrease of SO₂ from sulfur in the residual char burnt with air in the air reactor.     

Porous sunflower plate-like NiFe2O4/CoNi–S heterostructure as efficient electrocatalyst for overall water splitting

Constructing high-efficient and nonprecious electrocatalysts is of primary importance for improving the efficiency of water splitting. Herein, a novel sunflower plate-like NiFe₂O₄/CoNi–S nanosheet heterostructure was fabricated via facile hydrothermal and electrodeposition methods. The as-fabricated NiFe₂O₄/CoNi–S heterostructure array exhibits remarkable bifunctional catalytic activity and stability toward oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline media. It presents a small overpotential of 219 mV and 149 mV for OER and HER, respectively, to produce a current density of 10 mA cm^?2. More significantly, when the obtained electrodes are used as both the cathode and anode in an electrolyzer, a voltage of 1.57 V is gained at 10 mA cm^?2, with superior stability for 72 h. Such outstanding properties are ascribed to: the 3D porous network structure, which exposes more active sites and accelerates mass transfer and gas bubble emission; the high conductivity of CoNi–S, which provides faster charge transport and thus promotes the electrocatalytic reaction of the composites; and the effective interface engineering between NiFe₂O₄ (excellent performance for OER) and CoNi–S (high activity for HER), which leads to a shorter transport pathway and thus expedites electron transfer. This work provides a new strategy for designing efficient and inexpensive electrocatalysts for water splitting.