Assessment of kinetic model for ceria oxidation for chemical-looping CO2 dissociation
Introduction
Global warming constitutes currently one of the most discussed environmental issues, and the scientific community converges toward the awareness that the human impact on the environment is becoming more and more unsustainable. Most recognized responsible for such phenomenon are the greenhouse gases, mainly the carbon dioxide. One of the approaches to reduce CO2 emissions from fossil fuels is carbon capture and sequestration (CCS). Carbon dioxide can be separated from the emissions originated from fossil fuels combustion by using different technologies (e.g., absorption, adsorption, cryogenic distillation, etc.) and then sequestrated in geological formations or injected into nearly depleted oil/gas reservoirs for Enhanced Oil Recovery (EOR). Although several CCS technologies exist, there are significant challenges still associated with CCS, mostly due to safety and long-term stability and economic reasons [1]. The energy needs and environmental concerns, thus, drive to look after alternative approaches, such as CO2 splitting and/or utilization for the synthesis of fuels (e.g., methanol) [2], [3], [4].
Solar-thermochemical dissociation of CO2 into CO attracted significant interest after the initial success of thermochemical H2O splitting [5]. In fact, thermochemical cycles were initially focused on hydrogen production from water splitting using oxygen carrier materials. In a solar-thermochemical cycle, the oxygen carrier participates in two separate redox reactions, in which it is first thermally reduced (using solar energy) and subsequently oxidized by H2O or CO2. The thermal reduction (TR) step of the cycle is endothermic and requires a higher valence metal oxide, which releasing oxygen upon supply of external heat forms a lower valence oxide of the metal. In the second step, the reduced metal oxide is oxidized back to higher valence state by taking oxygen from H2O or CO2 to form H2 or CO [6]. Here, a reduction temperature higher than the oxidation one is the thermodynamic constraint for this process to be attainable, as shown in Fig. 1.
The net outcome of the cycle is the same as splitting H2O – or CO2 – into H2 and O2 – or CO and O2 – in a single step reaction, but compared to a single-step thermolysis reaction, it requires a sensibly lower temperature (e.g., water thermolysis takes place at above 2300 °C) and it also bypasses the problem of formation of explosive mixtures by producing separate streams of H2 or CO and O2 [7], [8].
In this work, ceria-based chemical looping for CO2 splitting is investigated. Ceria has been chosen as it is considered one of the most promising redox oxygen carriers because of its fast chemistry, high ionic diffusivity and large oxygen storage capacity. Using oxygen-deficient ceria, CO2 is dissociated into CO via:where δ is the non-stoichiometric oxygen capacity, corresponding to the surface oxygen vacancies determining the extent of CO2 dissociation (Eq. (1)). Vacancies are formed back in the reduction step by the release of oxygen (Eq. (2)). In solar-thermochemical CO2 splitting the oxygen removal step can be achieved either by heating ceria to a high temperature (∼1400 °C) using concentrated solar irradiation (Eq. (2)) or by reducing the oxygen carrier using H2 (or even other fuels, e.g. CH4), also called as reactive-chemical looping CO2 splitting (RCL-CDS).
The transfer of oxygen between the two redox steps exploits the non-stoichiometric oxygen capacity of the ceria, and the oxygen carrier remains intact at the end of the cycle. Though the redox cyclic process to produce syngas shows a promising potential, there are major challenges to achieve the high temperature required for the thermal reduction, which needs a concentrated solar plant (CSP) [9]. Apart from high heat demand, the large temperature swing between the two-steps renders the process to be less efficient if not designed properly.
Among many oxygen carrier materials for CO2 splitting, ceria has been widely investigated in experimental studies. A number of experiments [10], [11], [12], [13], [14], [15], [16], [17], [18] have demonstrated the feasibility of CO2 splitting with ceria, as listed in Table 1. However, only a few studies reported the reaction kinetics, mainly following equilibrium approach, defect model theory, empirical solid state kinetics models [19], [20], [21], [22], [23], [24], [25], [26], [27]. Arifin [27] has investigated the kinetics of splitting of water and CO2 over ceria and found that it is difficult to converge on a single kinetic model that adequately predicts the CO production behavior from thermally reduced ceria over the entire temperature range investigated. To achieve a high quality fit to the data, three separate models had to be used within the F family of models to give the best-fit to the CO transient signal with different kinetic parameters. Bulfin et al. [26] developed an analytical kinetic model to fit experimental data and found that R3 model gives the best fit results below 800 °C. Ackerman et al. [28] reported that D2 model provides the best-fitting for ceria oxidation at 1400 °C. The lack of agreement between the kinetic models based on various experimental studies is a point of observation. The difference of the reaction mechanisms adopted could be a consequence of variations in the experimental conditions or in the morphology of CeO2 samples. Therefore, the present work aims to statistically analyze the solid-state reaction kinetics models that describe the oxidation of non-stoichiometric ceria with CO2 by comparing their fitting goodness to a broad set of experimental measures. These reaction kinetic models are listed in Table S1 (Supplementary data) with related detailed formula.
In this work, CO2 dissociation over ceria is investigated by experiments and the measured reaction rates are used for kinetic models selection based on a statistical approach to identify the involved reaction mechanism.
Section snippets
Reaction activity testing
Isothermal redox cycles of CeO2 commercial powders are carried out in a horizontal tubular reactor in the temperature range of 700–1000 °C. H2 is used for the ceria reduction in order to explore the maximum non-stoichiometric capacity (δ) achieved at a set-point temperature while using a different concentration of carbon dioxide in the oxidation step. The temperature swing is thus replaced by isothermal operation for developing the kinetics. The CO production during the oxidation reaction is
Reactivity results
Firstly, the results of the tests have been investigated considering the CO2 splitting performance in terms of CO production rate (N ml/min/g) and total CO yield (N ml/g). With the purpose to highlight the dependence of the performance of the oxygen carrier on temperature and reactant gas concentration, the tests were carried in different experimental conditions, as described in the following sections.
Kinetic study of ceria oxidation
Generally, the reaction mechanism of solid state reactions is described using reaction-order models (F), geometrical contraction models (R), diffusion-limited models (D), nucleation models [31], [32], [33], [34] (also called as Avrami-Erofe’ev models, AE), random pore model (RPM) [35], and the Sestak-Berggren (SB) [36], and Prout-Tompkins models (PT) [37]. The schematic description of the general kinetic models is illustrated in Fig. S3 (Supplementary file). Generally, most of the models listed
Conclusion
This work presents a detailed kinetics study of CO2 splitting on ceria. The time resolved kinetics were measured in a horizontal tubular reactor at atmospheric pressure. The ceria sample was alternatively exposed to 5%H2 in Argon mixture in the reduction step to remove the lattice oxygen, and CO2 in the oxidations step to produce CO in the redox cycle. Tests were performed under isothermal conditions (700–1000 °C) for multiple redox cycles for three CO2 concentrations between 20% and 40% in
Acknowledgments
The research presented is performed within the framework of the Erasmus Mundus Joint Doctorate SELECT + program ‘Environomical Pathways for Sustainable Energy Systems’ and funded with support from the Education, Audiovisual, and Culture Executive Agency (EACEA) of the European Commission. This publication reflects the views only of the author(s), and the Commission cannot be held responsible for any use, which may be made of the information contained therein. J. Llorca is a Serra Húnter Fellow
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