First experimental studies of solar redox reactions of copper oxides for thermochemical energy storage

First experimental studies of solar redox reactions of copper oxides for thermochemical energy storage Elisa Alonso a,⇑ , Carlos Pe ´rez-Ra ´bago b , Javier Licurgo b , Edward Fuentealba a , Claudio A. Estrada b a Universidad de Antofagasta, Centro de Desarrollo Energe ´tico Antofagasta, Avda. Angamos 601, 1270300 Antofagasta, Chile b Instituto de Energı ´as Renovables, Universidad Nacional Auto ´noma de Me ´xico, Avda. Xochicalco s/n, A.P. 34, Temixco, 62580 Morelos, Mexico Received 25 January 2015; received in revised form 26 February 2015; accepted 2 March 2015 Available online 18 March 2015 Communicated by: Associate Editor Luisa F. Cabeza Abstract Thermochemical redox processes are currently considered one of the most promising methods for thermal storage of solar energy. Among the different types of materials available for this purpose, metal oxides allow higher operation temperatures in CSP systems. This is in agreement with the new R&D trends that focus on increasing the temperature to augment the efficiency. Copper oxide was previously proposed as a valid metal oxide for thermochemical storage. However, no demonstrative experiments had been carried out so far under solar radiation. In this work, the suitability of copper oxide was proved in a solar furnace. The employed solar reactor was a rotary kiln device with direct radiation absorption on reactive particles, which is a configuration that guarantees higher operation temperatures than other types of solar reactors. Given results include the performance of the CuO reduction in the rotary kiln under argon atmosphere and the cyclability of the pair CuO/Cu 2 O in air. Ó2015 Elsevier Ltd. All rights reserved. Keywords: Thermochemical storage; Copper oxide; Solar reactor; Solar furnace 1. Introduction CSP uses concentrated solar energy to generate electricity while producing very low levels of greenhouse-gas emissions. When combined with thermal energy storage capacity, CSP plants can still produce electricity when clouds block the sun or after sundown, what is a very significant advantage (Romero and Steinfeld, 2012; Zaversky et al., 2013). Thus, investigation on reliable and economically feasible thermal storage systems is currently one of the key challenges for the efficient and sustainable use of concentrating solar energy into the future (Agrafiotis et al., 2015; Cabeza et al., 2015; Galione et al., 2014; Karagiannakis et al., 2014). There are three main types of thermal energy storage (TES) technologies available: Sensible heat storage (SHS), latent heat storage (LHS) and thermochemical storage (TCS) (Gil et al., 2010). TCS is a relatively new technology with much research and development ongoing (Aydin et al., 2015). Its advantages and potentiality are certain. Energy stored density of TCS can be up to fifteen times higher than SHS and six times higher than LHS (Abedin and Rosen, 2011). Also, heat can be recovered at higher temperature and in a different range depending on the material employed to store. Finally, heat can be stored indefinitely in the form of chemical energy (Nagel et al., 2013). http://dx.doi.org/10.1016/j.solener.2015.03.005 0038-092X/Ó2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: [email protected] (E. Alonso). www.elsevier.com/locate/solener Available online at www.sciencedirect.com ScienceDirect Solar Energy 115 (2015) 297–305 The redox pairs of metal oxides have been proposed as a promising concept for thermochemical energy storage at high temperature. The system employs cycles of reduction and oxidation (redox) reactions to store and release heat. Operating temperature is in a range of 623–1373 K depending on the equilibrium temperature of the selected redox pair (Pardo et al., 2014). The general pathway of TCS based on metal oxides is: Reduction :MxOy!MxOy1þ1=2O2DH>0ð1Þ Oxidation :MxOy1þ1=2O2!MxOyDH<0ð2Þ Wong et al. (2010) and Wong (2011) analyzed sixteen oxides as possible candidates for this application. Among them, only BaO 2 ,Co 3 O 4 ,Mn 2 O 3 , CuO, Fe 2 O 3 , and Mn 3 O 4 demonstrated a suitable performance as TCS materials. Then, Co 3 O 4 was retained for further studies since it exhibited the best performance in terms of reversibility and energy storage density (Agrafiotis et al., 2014; Block et al., 2014; Neises et al., 2012). Neises et al. (2012) performed several cycles of reduction/oxidation irradiating the material with concentrated solar radiation. Mn 2 O 3 cyclability has been also recently proved by TGA tests and the influence of particle size has been analyzed (Carrillo et al., 2014). Relating to copper oxide pair (CuO/Cu 2 O), Wong (2011) indicated that re-oxidation of Cu 2 O seems to take place at temperature significantly lower than equilibrium transition. Since this capacity would provide flexibility in the design and operation of the storage system, they pointed out the potential of CuO as TCS candidate. Theoretical transition temperature is established in 1120 °C in air and the storage density is 811 kJ/kg (844 kJ/kg for Co 3 O 4 and 202 kJ/kg for Mn 3 O 4 ) according to Wong (2011). Moreover, copper oxide is widely available in areas of emerging CPS markets with very high solar resource, as Me ´xico and North of Chile, what could decrease its price compared with other TCS metal oxides. Apart from advanced TES materials, another key challenge of CSP concerns the increase of the thermal fluid temperature because it is favorable for improving thermal conversion efficiency. Investigation on this direction involves receiver materials and efficient designs or new thermal fluids proposals (Romero and Steinfeld, 2012). Hence, energy storage development should be also oriented to higher temperature systems. Metal oxides redox systems are conceived to be coupled with high temperature solar receivers where air is the thermal fluid. For example, volumetric receivers made of metallic or ceramic materials may increase the thermal fluid temperature above 1000 °C. In addition, recent investigations are focused on CSP tower plants with direct-absorption receiver using solid particles. Regarding to that, Ehrhart et al. (2014) proposed to combine the air-particles heat transfer media with solid thermochemical storage. According to their approach, a thermochemically active material is charged in a direct-absorption solar receiver/reactor by means of an endothermic chemical reaction. Then, the material is stored until heat must be recovered for the power block, when it is released through an exothermic chemical reaction. Among the different types of direct absorption particles solar reactors that were classified in Alonso and Romero (2015a,b), rotary kilns have the advantages of versatility, long life of components and low cost of maintenance. In fact, Neises et al. (2012) investigated the reduction and oxidation of Co 3 O 4 /CoO for thermochemical energy storage with a solar operated rotary kiln. For the reactor selection they justified that it favors the heat transfer between the gas and solid phase with a high reactive surface area. Moreover the particles motion should decrease their sintering. This feature is very interesting in case of working with copper oxides because the melting point of Cu 2 O is close to the transition temperature and sintering or partial melting could pose problems to the physical operation of the TCS system. Finally, the feasibility of solar heated rotary kilns for thermochemical application was shown in several cases (Chambon et al., 2010; Flamant, 1980; Meier et al., 2004). According to the above pointed out, in this study, the potential of the metal oxide redox pair CuO/Cu 2 O for TCS is evaluated, for the first time, under solar conditions with direct radiation absorption. A solar reactor was employed for experimental tests. It consists of a rotary cavity closed by a quartz window. The reactor was set up at the HoSIER solar furnace of the Renewable Energy Institute of the National University of Me ´xico (IER-UNAM). Experiences acquired with the first approach to the solardriven reduction of CuO into Cu 2 O are reported in this paper. Then, results of reduction/oxidation cycles in air according to the following reaction CuO $Cu2Oþ1=2O2ð3Þ are also presented and discussed. Nomenclature P RR power input to the solar reactor F f solar furnace factor Q i ideal power of the solar furnace X A chemical conversion of reactant “A” [O 2 ] instantaneous concentration of oxygen tgasflow in the reaction chamber qdensity of an ideal gas 298 E. Alonso et al. / Solar Energy 115 (2015) 297–305 2. Experimental set up and rotary solar reactor A rotary solar reactor was designed and installed at the HoSIER solar furnace in IER-UNAM, Me ´xico. The solar furnace consists of a 81 m 2 heliostat, and a 36 m 2 faceted mirror concentrator composed of 409 hexagonal facets with spherical surface and 30 facets with flat surface. A Venetian type shutter is located between the heliostat and the concentrator. Peak solar flux concentration ratios exceeding 18,000 suns can be achieved at the focal plane of the solar furnace and measured average irradiance was up to 5700 suns. More details about HoSIER can be found in Perez-Enciso et al. (2013), Estrada et al. (2011) and Riveros-Rosas et al. (2010). Fig. 1ashows a side view of the rotary solar reactor placed onto the test bench at the solar furnace focus and a drawn scheme that allows the identification of the different elements which the reactor is composed of. It consists of a ceramic cavity closed by a quartz window. The reactor body is stationary, what includes the stainless steel housing and an insulation layer of porous alumina. Only the reaction chamber rotates around a central axis what is connected to an engine at the reactor rear. Reaction chamber is a rotary cylinder made of alumina of 57.5 mm inner diameter, 67 mm outer diameter and 74 mm in length. In Fig. 1b, rotary parts are highlighted in grey. In order to avoid mechanical problems, bearings joining rotary and stationary pieces are water cooled. The reactor is provided with a water cooled quartz window of 210 mm diameter and 8 mm thick. Gas is injected into the reactor by four radial perforations equally spaced across a frontal cone that separates the window and the cavity. After passing the reaction chamber, carrier gas leaves the cavity through a duct placed at the reactor back. Temperatures at various locations of the reactor body were measured with T-type thermocouples. Among them, there were two thermocouples placed at the external wall of the reaction chamber. Although they did not exactly measured temperature inside the reaction chamber, their records were used as reference. In order to measure and record temperatures at rotary parts, and acquisition device is placed at the back at the reactor. It rotates together with the reactor and it is connected by wireless signal with a computer. Inlet gas flow rates were controlled using electronic flow meters Bronkhorst HI-TEC. The composition of the product gases was monitored by an electrochemical based detector for O 2 Siemens Oxymat 64. The gas analyzer includes a filter that detains moisture to avoid damaging the detector. Fig. 2 shows the experimental set up scheme. 3. Experimental procedure Before the experimental campaign, it was necessary to determine the operating conditions of the HoSIER solar furnace. The power input to the solar reactor at every moment is: PRR ¼FfDNI 1000 %shutter Qið4Þ where Qiis the ideal power of the solar furnace (28.5 kW according to Perez-Enciso (2015)) and F f is the solar furnace factor. It was experimentally measured during a sunny day on the same month when the most of the present tests were carried out (Bliss, 1957; Perez-Enciso et al., 2013). F f represents the percentage of radiation reflected by the heliostat that reached the focal zone of the furnace when the shutter was completely opened. The rest of the radiation was blocked by the side face of the shutter sheets and the devices placed between the shutter and the concentrator. The starting reactant was, in every experimental case, 10 g of CuO 98% powder (particles size <10 lm) of Sigma Aldrich. Once the reactor was properly placed at the focal zone, it was opened by removing the window and the reaction chamber was filled with the sample. Then the reactor was closed again and cooling water, gas flow and rotation engine were switched on. Gas and water cooling flows and rotation speed were maintained constant during the whole experiment, including, heating and cooling steps and also the cooling after the end of the experiment. Fig. 1. (a) Photograph of the rotary solar reactor installed in the focal zone of the solar furnace (window cooling circuit removed). (b) Sketch of the solar reactor with rotary parts marked in grey. E. Alonso et al. / Solar Energy 115 (2015) 297–305 299 Employed gases were argon (99.99% purity) and air. In both cases, some minutes were required to stabilize the oxygen concentration at the reactor outlet. In case of argon, all the air had to be purged from the reaction chamber until the oxygen analyzer exhibited a zero baseline. Operation values of gas flow rate and rotation speed were 10 Nl/min and 4 rpm respectively. Both values were selected as conclusions from several initial tests of the reactor performance. Also, literature was revised for selecting the flow rate (Schunk and Steinfeld, 2009; Alonso et al., 2013; Alonso and Romero, 2015a,b). Thus, examples of other cavity-type solar reactors for gas–solid thermochemical reactions were taken into account to select a magnitude order. It was experimentally checked that 10 Nl/min allowed a rapid purge of the reaction chamber and stabilization of the oxygen concentration. With that flow, small overpressure (millibars) was always kept in the reaction chamber. That secured no air from atmosphere would have entered through the window seals or tubing. During experimental campaigns with sun, it was proved that the flow rate was adequate to appreciate with enough accuracy any oxygen variation resulting from chemical reactions. Regarding rotational speed, it was observed that 4 rpm was adequate to allow the desired particles movement into the cavity. By the other hand, if higher speed was imposed, some particle left the reaction chamber reaching the quartz window, what was not desirable. Once the oxygen concentration was stationary, heating was started by opening the shutter. The incident power was regulated by the opening percentage, which in turn offset the irradiance variations. Reductions in argon required a just heating strategy. However, to achieve reduction– oxidation cycles, heating and cooling have to be intercalated. Thus, shutter was completely closed in cooling steps. Opening percentage variations were manually set according to the observed operation parameters. A control station formed by several panels allowed a constant tracking of the whole process. Several temperatures at the reactor, including the reference temperatures, radiation power and oxygen evolution at the reactor outlet were monitored and recorded during the whole experiment. Temperatures at the rotary reaction chamber were recorder by the wireless acquisition system with a time step of 3 s. Other temperatures at the reactor body (housing, insulation, window, etc.) were registered with a time step of 1 s. After the end of an experiment, the reactor was opened when reference temperature had decreased below 400 °C and the reactive material was extracted. Crystallographic phases in the solid products were identified by powder Xray diffractometry with a Rigaku X-ray diffractometer (Mod. DMAX-2200). 4. Results and discussion Several experimental cases were carried out according to the experimental procedure described below. Tests are differentiated in thermal reduction experiments and redox cycle experiments. 4.1. Reduction of CuO in argon During the whole experimental campaign the rotary solar reactor operated reliably and withstood thermal shocks and high temperatures including the rotary parts and rotary-static joints. Rotational speed was adequate to allow the particles rising adhered to the cavity wall and then dropping down in front of the radiation beam. Fig. 2. Scheme of the experimental set up including the concentrator, the solar reactor, the gas line, the cooling water circuit and the data acquisition system. 300 E. Alonso et al. / Solar Energy 115 (2015) 297–305 Such a performance was observed by a video camera that recorded continuously the front side of the solar reactor. Moreover, the window remained transparent with no deposition of powder on it. Due to the stir generated, conditions tended to homogenize inside the reaction chamber as the experiment progresses. In some pre-campaign experiments, the solar reactor performance had been proved without rotation. Metal oxides samples had been arranged in a stationary packed bed using a rod as sample-holder. A similar configuration can be found in Alonso et al. (2013) and Schunk and Steinfeld (2009). This way, radiation impinged on the sample front surface and heated the material. However, there were found high temperature gradients along the sample. To achieve the reaction temperature at the rear of the sample, it was needed to increase radiation power at the front, what often led the sample melting. Assuming that melting was not desired, power had to be carefully controlled and total yields higher than 20% were unable to obtain at the end of the thermal treatment, even after long periods of twice or three times the duration of a typical rotary experiment. With a rotary performance, the solar reactor showed more suitability for the reduction of metal oxides which solid products have to be used afterwards. Fig. 3 a shows the appearance of the powder CuO inside the reaction chamber before the thermal treatment and Fig. 3b the product just after the heating was stopped. Although some particles lightly adhered to the reactor wall, neither fusion nor sintering was detected. Once the reactor was cooled down, the particles were easily peeled off and almost all of the material could be recovered. In Fig. 4 a it is shown the sample after reducing in argon with the rotary reactor. The particles formed small spheres or balls with a range of diameter about 0.5–3 mm. They presented a weak compaction so that the original consistence of the particles could be restored with a soft compression. The color of the product was close to the 1 red of Cu 2 O. In order to corroborate the composition of the powdered product, it was examinated by XRD characterization. Fig. 5 shows the pattern of the sample of a typical CuO reduction in argon and its comparison to the CuO and Cu 2 O peaks of diffraction. It shows the obtained product is a mix of Cu 2 O and the CuO which was not reduced. Chemical conversion (X A ) at the end of the experiment (when heating was stopped) was calculated according to Eq. (5). It takes into account the accumulative amount of oxygen delivered until the end of the experiment. XA¼Rt 0½O2vqdt amA cPMAPMO2 ð5Þ where “A”refers to CuO, and aand care the stoichiometric coefficients of CuO and O 2 respectively in Eq. (3).[O 2 ]is the instantaneous concentration of oxygen recorder by the O 2 analyzer, tis the gasflow in the reaction chamber and q is the density of an ideal gas. Conversion of almost 80% was obtained for the most favorable CuO reduction. For that, it was required treatment time of more than 3700 s and maximum reference temperature reached was around 900 °C. Figs. 6 and 7 refer to two different experiments of CuO reduction in argon. Parameters they show are the power input in the rotary reactor (P RR , acording Eq. (4)), the oxygen delivered by the sample and two reference temperatures. The oxygen curve exhibits the reaction evolution while the thermal treatment endured. Temperatures were measured at two different points of the cylindrical external wall of the reaction chamber, with a gap of around 3 cm. Small differences are appreciate between both temperatures. These temperatures are useful as a reference of how fast or slow the system was heated, although they do not represent the real reaction temperature at the sample. However, knowed that Cu 2 O melts at 1235 °C and this did not ocurr in any case, it can be presumed that reference temperature was less than 300 °C lower than reaction temperature. Anyway, since reaction temperature is actually one of the main uncertainties of this type of solar driven processes (Alonso et al., 2013; Kogan, 2003) a deep thermal analysis of the reactor, including the reactant material, is planned for next future by means of a CFD model. Experiments corresponding to Fig. 6 and 7 were carried out in different days. The power and, therefore, the temperature was regulated by the shutter opening in order to accomplish a progressive temperature increasing until the oxygen releases comprehensively. During the first one there were only occasional clouds that caused power and temperature drops. The oxygen delivery started when reference temperature was about 500 °C. The concentration curve shows very irregular shape with pronounced rises and drops instead of one uniform peak as observed in other published works of solar driven redox processes (Alonso et al., 2013; Neises et al., 2012; Chueh et al., 2010). This could be related to a strong influence of the power fluctuations on the particles reactivity. In addition, small temperature differences at the reaction chamber walls as well as sample in continuos movement may originate the delivery of oxygen from different reactive zones. In case of Fig. 7, a layer of thick clouds appeared after 1800 s of operation resulting in a sudden drop of power that cut off the reaction. In a first stage, temperature is increased faster than showed in Fig. 5, however, oxygen delivery also started at reference temperature close to 500 °C. When clouds appeared and after almost 1000 s without radiation, reference temperatures decreased from 800–850 °C to 400 °C. Then, the sky got cloudless again and a second stage of thermal treatment was carried out. As soon as the power into the reactor was reestablished, temperature started to rise again. Also oxygen started releasing at the same temperature than before. This 1 For interpretation of color in Fig. 4, the reader is referred to the web version of this article. E. Alonso et al. / Solar Energy 115 (2015) 297–305 301 indicates that, despite an intermittent heating, the reduction progresses while enough power is feeding the reactor, until a maximun value of around 80% at the end of the thermal treatment for the selected operation parameters (see Fig. 7). The other 20% of non-active material could be also reduced after an optimization of the reaction parameters. Rotational speed and gasflow could be adjusted taking into account the feasible operation ranges. Also, particles size and initial mass of reactant are the main parameters which, in further analysis, would be subject to assessment. 4.2. Redox cycle in air Once the suitability of the solar reactor was proven for thermal reduction of CuO in argon, some cycles of reduction–oxidation in air were carried out. Again, the shutter opening percentage allowed the control of the power entering in the reactor and the temperature of the system. The followed strategy was to increase the temperature in order to reduce CuO into Cu 2 O and cool down to re-oxide the sample towards CuO. Maximum and minimum values for Fig. 4. (a) Sample after a reduction experiment in argon. (b) Sample after a cyclability experiment in air. Fig. 3. (a) Reactant and product in the reaction chamber before and (b) after the thermal treatment. Fig. 5. XRD pattern of the sample reduced in argon. 0 5000 10000 15000 20000 25000 0 100 200 300 400 500 600 700 800 900 1000 0 1000 2000 3000 4000 Oxygen (ppm), Power·10 (W) Temperature (ºC) Time (s) Temperature 1 Temperature 2 Oxygen Power Fig. 6. Power, reference temperature and oxygen evolution curve of a typical argon reduction experiment. 302 E. Alonso et al. / Solar Energy 115 (2015) 297–305 reference temperature were remained about 1000 and 700 °C. Fig. 8 shows a test of three redox cycles that was selected as a typical case. A baseline of oxygen concentration corresponds to the carrier air flow. Firstly, oxygen was delivered during reduction and concentration rose. Reference temperature when oxygen emerged was around 800 °C, i.e., 300 °C higher than in case of reducing with argon. Moreover, the maximum reduction conversion obtained under air atmosphere for the most favorable case was only 40%, while it was 80% in presence of argon. These results are predictable because thermal reductions of oxides are usually favoured by low oxygen pressure or inert atmosphere instead of air as carrier gas (Maruga ´n et al., 2012). An irregular shaped curve was also generated by the augmentation of the oxygen concentration in this case. In fact, the second reduction peak was so fluctuating that sometimes decreased under the baseline. That could be caused by the also fluctuating power, which could be inducing partial oxidations when sudden and uncontrollable power drops happened. The reduction was considered ended when the oxygen concentration had reached again its initial concentration. At this moment 0% was manually imposed in the shutter opening percentage and oxidation started while temperature was decreasing in absence of any heating. As observed in Fig. 8, oxidation took place as soon as power input was cut off, even though the reference temperature had not decreased yet. However, at reaction zone, presumably, the temperature decreasing was much faster due to the convective effect of the air crossing the reaction chamber and the re-irradiation towards the window. By integrating the oxygen peaks, reduction and oxidation conversions can be obtained as the relation between the amount of oxygen delivered or consumed and the maximum amount corresponding to the available material (see Eq. (5)). While maximum reduction conversion was around 40%, only 9% was achieved for oxidation. It was also observed that the size of both peaks decreased with a new redox cycle, what means that the fraction of active material was reducing. This may happen because the morphology of the powder changed with the thermal treatment. Fig. 4b shows the appearance of the material after three redox cycles. In contrast, Fig. 4a shows the product of a reduction in argon, what had led to the creation of small non compacted spheres. In that case, spheres could be easily broken to recover the initial consistence of the material because they were not sintered. However, after a cyclability test, harder balls were extracted from the reactor indicating that stronger agglomeration and sintering had occurred. It should be noted that the maximum temperature is around 100 °C higher than for reduction in argon. Hence, sintering may hinder the oxygen transfer between the solid material and the air. The color of the product in Fig. 4b is black with just some particles slightly red shaded. It is different from the product in Fig. 4a, and closer to the color of CuO than Cu 2 O. XRD characterization allowed the identification of a mix of CuO and Cu 2 O with more marked CuO diffraction than those shown in Fig. 5 (Fig. 9). Results indicate CuO/Cu 2 O cyclability is feasible by heating with solar radiation. In a second experimental campaign, operation parameters should be optimized in order 192000 194000 196000 198000 200000 202000 204000 206000 208000 210000 212000 0 500 1000 1500 2000 2500 3000 3500 4000 0 1000 2000 3000 4000 Oxygen (ppm) Temperature (ºC), Power (W) Time (s) Temperature 1 Temperature 2 Power Oxygen Fig. 8. Power, reference temperature and oxygen evolution curve of a cyclability experiment in air. 0 5000 10000 15000 20000 25000 30000 0 100 200 300 400 500 600 700 800 900 1000 0 1000 2000 3000 4000 Oxygen (ppm), Power · 10 (W) Temperature (ºC) Time (s) Temperature 1 Temperature 2 Oxygen Power Fig. 7. Power, reference temperature and oxygen evolution curve of an argon reduction experiment with the appearance of thick clouds. Fig. 9. XRD pattern of the sample cycled in air. E. Alonso et al. / Solar Energy 115 (2015) 297–305 303 to increase the active fraction of material and to avoid the loss of storage capacity. First, reduced fraction should be augmented up to 80% as suggested in previous section. Then, sintering should be avoided with a finer control of the reaction temperature. This way, it is expected that all the reduced material can be reconverted and completely reduced again. Hence, in further works it will be developed a procedure to measure and finely control reaction temperature by means of non-contact methods. 5. Conclusions A rotary solar reactor was employed to prove the suitability of the pair CuO/Cu 2 O as material for thermochemical energy storage. The performance of the reactor was appropriate for the thermal treatment of copper oxide particles in a high flux solar furnace. Thermal control at the reaction chamber was accomplished and the material melting was prevented. Rotation movement helped to improve the reactiveness of the sample by mixing the particles. Reduction of CuO in argon atmosphere led to conversions of almost 80%. It was observed that the oxygen delivery started at a fix reference temperature. However, power fluctuations affected the intensity of the oxygen release. Although the rotation movement generated the particles grouping in small spheres, the consistence of the material did not change in case of reduction with argon. In a second stage, several cycles of reduction and oxidation were accomplished in air. Reduction was disfavoured by the presence of air, in comparison to argon. In this case stronger coalescence of the particles was found, what may handicap the good progression of the redox reactions. It was found that the size of the oxygen peaks corresponding to the reduction and oxidation decreased with a new redox cycle. Also, the chemical conversion was significant lower for the oxidation than for the reduction. An optimization of the operation parameters such as rotational speed, particle size, gasflow and initial mass of reactant is proposed for further studies in order to increase the conversion of both, reduction and oxidation, and to avoid the progressive loss of reactive fraction. 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