Two step metal oxide thermochemical redox cycles have seen growing interest in the research community over the last two decades. In particular, they have often been studied as a means of converting ...heat from concentrated solar power to chemical energy, which can subsequently be used for a number of applications. In this work, we offer critical perspective and valuable insight on these research fields, from authors with a combined experience of 50+ years in metal oxide redox cycles. The current fundamental understanding of thermochemical redox materials, and the implications and limitations this has on the redox thermodynamics are discussed. The underlying fundamentals of the redox materials are then used to give insight into the theoretical limitations imposed on a number of applications including; solar thermochemical fuel production, solar energy storage for off sun power generation, thermochemical air separation, oxygen pumping, and thermochemical heat pumping. A number of recent novel process developments are also presented, which offer valuable motivation and direction for the respective fields. The analysis shows that oxides which undergo a stoichiometric phase change during reduction, such as ZnO or Co
3
O
4
, have much larger specific energy storage than materials undergoing partial reduction such as ceria and perovskites. The partial reduction materials generally have faster kinetics and better activity at low temperature, and the selection of materials for the various applications is often a compromise between the importance of high specific energy storage
vs.
fast kinetics and low temperature operation.
A critical review of thermochemical redox cycles as a means of converting heat to chemical energy and subsequent applications.
In this work, we perform a technical analysis of a cyclic reduction and oxidation process of metal oxides applied as a hydrogen storage route. Hydrogen is used to reduce a metal oxide and form steam, ...and regenerated by oxidizing the reduced metal oxide with steam. The reduced metal acts as the solid storage medium, which could be transported or used for stationary applications. Suitable metal oxides were screened according to the following performance indicators: specific energy demand (enthalpy change of reaction), thermodynamic conversion extents, specific storage density, resistance to sintering (melting point), and material cost. Iron oxide was then selected as the most promising candidate. A thermodynamic model was developed to determine favorable process temperatures of >400 °C for the hydrogen storage step and 100–500 °C for the hydrogen release step. The energy demand of the storage was calculated to be 27% of the lower heating value of hydrogen. Vis-a-vis other hydrogen storage methods, the iron-based storage exhibits drawbacks for on-board mobile applications but could be attractive for large-scale and long-term storage applications.
•Comparison of methane reforming via catalytic and redox chemical looping routes.•Phase equilibrium methodology for solid gas reactions to model a fixed bed.•A case study of both solar reforming ...routes coupled to a methanol synthesis plant.
Thermochemical methane reforming to syngas is performed on a massive scale in the chemical industry, providing feedstock for many chemical processes such as hydrogen, ammonia and methanol production. The high temperature process heat required for the endothermic reforming reaction could be supplied by conentrated solar energy, in a hybrid solar-fossil process. This can be achieved by re-designing conventional reforming technologies to utilize solar energy as the heat source. Another possible approach is to use a two-step metal oxide redox cycle. Here we compare the two solar thermochemical reforming routes, namely redox reforming and catalytic reforming using thermodynamic analysis and discuss the prospects for both technologies with a focus on methane conversion extents, syngas composition, and energy conversion efficiencies. Further processing of the syngas to liquid fuels is also discussed, in order to highlight how these processes can fit together with gas-to-liquids technologies. The analysis highlights that the redox cycle approach could produce a higher quality syngas, but at the expense of additional thermodynamic constraints, which are sensitive to carbon formation, and also lead to a greater energy demand relative to catalytic reforming.
Perovskites AMO3- delta are ideal for thermochemical air separation due to their oxygen nonstoichiometry delta , which can be varied by changing the temperature and oxygen partial pressure. We show ...in this work how materials can be selected for chemical looping air separation from thermodynamic considerations and present thermogravimetric experiments carried out on (Ca,Sr) ferrites and manganites, and doped variants, all synthesized via a citric acid auto-combustion method. SrFe0.95Cu0.05O3- delta and Ca0.8Sr0.2MnO3- delta show the best gravimetric oxygen storage capacity of all tested materials at T < 1200 degree C. The redox reactions are completed in <1 min in air and are highly reversible. A significant re-oxidation reaction of reduced samples was observed at temperatures as low as 250 degree C at an oxygen partial pressure of 0.16 bar. We studied phase formation via XRD and lattice expansion during reduction via in situ XRD experiments. The objective is to validate the potential and boundary conditions of such materials to pave the way for competitive air separation based on thermochemical cycling.
Perovskite oxides CaMnO3 and Ca0.8Sr0.2MnO3 show continuous non-stoichiometry over a range of temperatures and oxygen partial pressures. In this work a thermobalance equipped with an oxygen pump was ...used to measure the equilibrium non-stoichiometry of both materials for temperatures in the range 400-1200 degree C and oxygen partial pressures in the range 1-10-5 bar. Analysis of the data showed that Ca0.8Sr0.2MnO3 has a lower enthalpy of reduction and thus can be more easily reduced. The strontium added sample was also robust against a phase transition that was seen in CaMnO3 at high temperatures. A statistical thermodynamic model of the system suggests that the defects form clusters of the form . The oxidation kinetics were also investigated with Ca0.8Sr0.2MnO3 showing faster kinetics and maintaining activity at lower temperatures. Overall, Ca0.8Sr0.2MnO3 shows very promising properties for redox applications, including gravimetric oxygen storage up to 4% by mass, high stability and rapid reversibility, with re-oxidation in less than 1 min at 400 degree C. Finally, the redox chemistry of Ca0.8Sr0.2MnO3 was also investigated using in situ X-ray photoelectron spectroscopy and near-edge X-ray absorption measurements at near ambient pressure in oxygen atmospheres.
In this work the thermodynamics of thermochemical fuel production using a CeO2 redox cycle are studied. The need to reduce the oxygen partial pressure in order to improve efficiency is investigated, ...with both sweep gas and vacuum pumping considered as methods of achieving this. At ambient pressure the cycles can be maximized with respect to the temperature swing, the minimum oxygen partial pressure, and the extent of the oxidation reaction. For reduction at 1500 °C the maximum efficiency was found to be 4.5%, which is significantly lower than the values found in previous studies. In addition isothermal operation had very low efficiency (less than 2%) under all of the conditions considered. If the system is operated at lower than ambient pressure, the pumping efficiency will depend on the pressure. From an investigation of commercially available pumps the pressure dependence was given an analytical expression. The results showed the cycles have an optimal operating pressure and that using sweep gas, as well as pumping, only reduced the overall efficiency. The efficiency was maximized with respect to the temperature swing, the reduction pressure, and the extent of oxidation, giving a peak efficiency of 7.5% for a reduction temperature of 1500 °C. Reducing the pressure during reduction could also be beneficial due to improved reaction kinetics at lower pressure and an increased yield due to lower oxygen partial pressures. Recovering heat from both the high temperature ceria and the oxidation reaction, and using it as process heat, was also considered. With 60% of this heat being recovered, the peak efficiency for the 1500 °C pumped cycle increased to 11%. Finally the practicality of the cycles, in terms of the quantity of ceria required to maintain continuous operation, are considered, and some suggestions for improving the cycle are given.
In this work, an Arrhenius-based model for the high-temperature reduction and oxidation of CeO2 is developed. The model is shown to agree well with both literature data for the equilibrium oxygen ...vacancy concentration and novel experimental kinetics of oxidation and reduction obtained by the authors. The form of the Arrhenius rate equation was determined from the properties of the reaction. Equilibrium data from the literature was analyzed with respect to our rate equation. From this analysis, a number of constraints on the model parameters were determined, and some of the constants of the model were fixed. The model accurately predicts the equilibrium composition of CeO2 over a wide range of oxygen partial pressures (10–2 to 10–8 bar) and temperatures (1000–1900 °C). Novel results of the experimental reoxidation of ceria were analyzed to fix the remainder of the constants. Porous cerium dioxide pellets produced by the authors were reduced at high temperature (1650 °C) and low oxygen partial pressure (10–5 bar). The reduced cerium pellets were then reoxidized in an oxygen atmosphere of 1.4 × 10–4 bar at temperatures in the range 500–1000 °C. The reoxidation was conducted in a sealed vacuum chamber. The reaction was monitored via the change in pressure and gas composition measured by a manometer and mass spectrometer. The results from this reoxidation experiment allowed us to fix the values of the activation energies and frequency factors of the oxidation and reduction. The model was then compared with experimental reaction kinetics of thermal oxidation and reduction and showed good agreement.
The oxidation and reduction of mixed cerium zirconium oxides, Ce1–x Zr x O2, with x varied in the range 0–0.3 in steps of 0.05, in a low-pressure oxygen atmosphere were experimentally investigated. ...The experiments utilized a novel method developed by the authors, in which the samples are reduced and oxidized in a sealed vacuum chamber by irradiating them with a xenon arc lamp. The changes in pressure due to the release or consumption of oxygen are used to quantify the extent and rates of the reactions. The system can achieve heating rates in excess of 100 °C s–1, which is hundreds of times faster than is possible with a thermobalance. In addition, the gas phase transport is very rapid as it is driven by pressure gradients. The combined high heating rate and fast gas phase transport offers unique conditions in which to measure the actual kinetics of the reactions. The oxidation kinetics showed a strong dependence on the amount of Zr4+ in the samples, where the rate of oxidation decreased with increasing Zr4+ concentration. The effect of adding 2.5% Sm3+ or La3+ to the Ce0.85Zr0.15O2 was also investigated with the hope of improving the kinetics. The addition of La3+ to Ce0.85Zr0.15O2 was seen to offer an increase in the reaction rates, although the oxidation kinetics were still slower than that of pure CeO2. Finally, the reduction kinetic data are presented, but even with the optimal conditions the reduction reaction still appeared to be limited by the heating rate; thus, it was concluded that the reduction kinetics were too rapid to quantify by this method.
The perovskite oxide SrFeO3 has favourable redox properties for oxygen exchange applications, including oxygen separation and oxygen production chemical looping cycles. For such applications, lower ...temperature operation can improve the energy demand and feasibility of the process, but can also lead to kinetic limitations. Here we investigate the oxidation and reduction reaction kinetics of SrFeO3 in the temperature range 450–750 K. Isothermal relaxation techniques are used to observe the reaction rates across this temperature range, using a thermogravimetric analysis system. Experimental data are analysed according to an isoconversional method and fit with a simple power law model to extract activation energies. The apparent activation energy of oxidation and reduction was found to be 92 ± 16 and 144 ± 17 kJ mol−1 respectively. Comparison of oxidation and reduction kinetics together with considerations of particle size indicate that the oxidation reaction rate may be limited by diffusion in the bulk, while the reduction reaction rate is limited by the surface reaction. Furthermore, we also investigated the mixed perovskite Sr0.93Ca0.07Fe0.9Co0.1O3, which exhibited a 4-fold increase in the oxidation rate.
The thermodynamic redox properties of ceria and ceria zirconia solid solutions are analysed with a new methodology for modelling such systems based on the statistical mechanics of lattice ...configurations. Experimental thermogravimetric equilibrium data obtained for small non-stoichiometry measurements are combined with literature data to cover a large range of non-stoichiometry (CeO2-δ, δ = 0.001-0.32), temperature (1073-1773 K) and oxygen partial pressure (1-10(-13) bar). A dilute species model of defect clusters , obeying the law of mass action, was sufficient to describe the system over the whole range of conditions, leading to a simple analytical equation of state for the system. This offers new physical insight into the redox properties of ceria based materials, and the theoretical methods developed should also be of great interest for other materials which exhibit continuous oxygen non-stoichiometry similar to ceria, such as perovskite oxides.
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