Contents lists available at ScienceDirect Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat Modeling of double-loop fluidized bed solar reactor for efficient thermochemical fuel production Marco Milanese ⁎ , Gianpiero Colangelo, Fabrizio Iacobazzi, Arturo de Risi Department of Engineering for Innovation, University of Salento, SP per Monteroni, Lecce, Italy ARTICLE INFO Keywords: Fluidized bed Solar reactor Syngas Cerium oxide Nanoparticles ABSTRACT A new model of solar reactor based on a double-loop fluidized bed involving CeO 2 nanoparticles and two gas streams, N 2 and CO 2 , for efficient thermochemical fuel production, is presented. The fluidized bed reactors are commonly used to carry out a variety of chemical reactions, due to solid granular materials, which play the fundamental role of catalyst. In the system under investigation, the overall reaction CO 2 →CO+1/2O 2 is achieved, by means of a thermochemical two-step cycle, based on CeO 2 nanoparticles. The first step (CeO 2 thermal reduction) has been implemented with a solar-driven endothermic dissociation of the metal oxide to lower- valence metal-oxide. The second step (CO 2 splitting) has been carried out with an exothermic oxidation of the reduced metal-oxide, which is produced in the first step, to form CO. The use of nanoparticles as catalyst allows maximizing the surface area of reaction, and at the same time, the reactor based on double-loop fluidized bed allows continuous operation, without alternating flows of inert sweep gas and CO 2 . The thermodynamic analysis of the system under investigation showed a calculated maximum ideal efficiency of about 63%. 1. Introduction According to the Intergovernmental Panel on Climate Change, atmospheric concentration of carbon dioxide (CO 2 ) has to be stabilized at or below 450 ppm [1] and global CO 2 emissions must be reduced of 50% from 2006 levels by 2050, 100% by 2075, and beyond 100% by 2100 [2]. In order to reach these goals, in the next decades, all CO 2 sources have to be minimized, as well as the removal of CO 2 from the atmosphere will be necessary. In this scenario, syngas, that is a mixture of H 2 and CO, can represent one of the most promising sustainable energy carriers when produced from renewable resources [3]. Today, syngas production methods include mainly steam reforming of natural gas or liquid hydrocarbons to produce hydrogen and gasification of coal, biomass or waste [4,5]. Furthermore, several processes of syngas production, based on solar energy as heat source, have been developed in the last decades [6,7]. Particularly, the thermochemical two-step cycles, based on metal oxides, represent a valid option for syngas production, due to their lower temperature ( < 1700 K) compared to other processes. In this kind of process, the first step (thermal reduction) is developed through a solar-driven endother- mic dissociation of the metal oxide to elemental metal or lower-valence metal-oxide. The second step (CO 2 splitting) is constituted by an exothermic oxidation of the reduced metal-oxide, which is produced in the first step, to form CO [3]. The overall reaction of the two-step cycle is the following: CO →CO+1/2O 2 2 (1) In order to select the best metal-oxide, able to improve the reaction (1), many materials have been investigated, such as ferrites, zinc oxide, etc. [8–10]. According to Chueh et al. [11], the cycles based on ferrites suffer the sintering process and present slow kinetics [12–19], as well as zinc oxide based cycles require rapid quenching because of volatilization [20–22]. Recent studies on thermochemical dissociation of CO 2 focused their attention on ceria (CeO 2 ) as active material for CO 2 splitting reaction, because it has an extremely high melting temperature of approximately 2800 K and shows high catalytic activity towards carbon-containing gases [23–28]. Cerium Oxide is largely investigated in literature for its structural, chemical and optical properties that make it a promising material in several fields of applications, such as gas sensing, high refractive index material, fuel cells, catalysis, CO 2 adsorbing materials, nanofluids etc. [29–31]. Furthermore, Milanese et al. [32] demonstrated that the CeO 2 preserves its optical properties, even after several runs of thermal processes. With the purpose to use solar energy as heat source, coupled with the above-described thermochemical two-step cycle, in the last years several innovative reactor designs have been proposed. Haueter et al. http://dx.doi.org/10.1016/j.solmat.2016.10.028 Received 7 July 2016; Received in revised form 17 October 2016; Accepted 18 October 2016 ⁎ Corresponding author. E-mail address: marco.milanese@unisalento.it (M. Milanese). Solar Energy Materials & Solar Cells 160 (2017) 174–181 0927-0248/ © 2016 Elsevier B.V. All rights reserved. Available online 27 October 2016 crossmark