CERAMICS INTERNATIONAL Available online at www.sciencedirect.com Ceramics International 39 (2013) 5459–5465 Assessment of redox behavior of nickel ferrite as oxygen carriers for chemical looping process Yu-Lin Kuo a,n , Weu-Mau Hsu a , Ping-Chin Chiu b , Yao-Hsuan Tseng b , Young Ku b a Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan b Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan Received 17 September 2012; received in revised form 27 November 2012; accepted 14 December 2012 Available online 26 December 2012 Abstract This study investigated the suitability of using nickel ferrite (NiFe 2 O 4 ) oxygen carriers for a chemical looping process. NiFe 2 O 4 powder was prepared by ball milling equimolar NiO and Fe 2 O 3 in a high temperature solid-state reaction. Material characteristics of NiFe 2 O 4 samples were investigated by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area measurements, and scanning electron microscopy (SEM). Redox cycling of NiFe 2 O 4 oxygen carriers was performed by thermogravimetric (TGA) measurement under pure CH 4 gas and O 2 /Air atmospheres, respectively. After five successive cycles, NiFe 2 O 4 powder with a single phase of spinel structure demonstrated higher redox cycling behavior and better stability than standard NiO and Fe 2 O 3 . We also addressed the mechanism underlying the redox cycling by NiFe 2 O 4 spinel powder. Our results demonstrate the feasibility of using the proposed preparation of NiFe 2 O 4 as an oxygen carrier in a reversible chemical looping process (CLP). & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Nickel ferrite; Redox cycling; Oxygen carriers; Chemical looping process 1. Introduction Carbon dioxide (CO 2 ) generated by the combustion of fossil fuels is the greenhouse gas primarily responsible for global warming [1]. The chemical looping process (CLP) has been proposed as a cost effective scheme for separating CO 2 from the emissions produced by burning coal [2–5]. Lewis and Gilliland patented the production of pure carbon dioxide using metal oxides as a means to process the cyclic reduction and oxidation (redox) reactions in two interconnected fluidized beds [6]. CLP involves the use of metal oxides as carriers to transport lattice oxygen to react directly with fuels during reduction, thereby avoiding direct contact between fuel and air. During oxidation, the reduced metal oxide is re-oxidized to its original form by combustion in air. The primary benefit of CLP technology is the production of pure CO 2 and stream (H 2 O) without the need to expend energy in the separation of CO 2 and N 2 . In addition, the CLP process associated with cyclic redox reactions is exothermic [7]. Selection of the oxygen carrier is a key step in the development of CLP technology to ensure continuous reaction in the fuel reactor and air reactor. Previous researches into the thermodynamic properties, reactivity, recyclability, and mechanical strength of oxygen carriers have indicated the applicability of Fe, Ni, Cu, Mn, and Co oxides as candidates in CLP technology [5–10]. Fan et al. [5,11] compared key properties of various metal oxide candidates, determining that iron-based oxygen carriers possess higher oxygen carrying capacity, a higher melting temperature, and suitable mechanical strength, making them a favorable choice for CLP. However, the less poor reactivity of a-Fe 2 O 3 with fuels is considered a complicated gas–solid reaction due to the several structural changes involved [12–14]. Nonetheless, the appropriate doping of metal oxides to form composites is viewed as a promising approach to increasing reactivity [15–18]. Shimokawabe et al. [14] reported the applicability of 13 metal oxides as doping agents to alter the reactivity of a-Fe 2 O 3 . The results of TGA testing on the hydrogen www.elsevier.com/locate/ceramint 0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2012.12.055 n Correspondence to: No.43, Section 4, Keelung Road, Taipei 10607, Taiwan. Tel.: þ886 2 27376784; fax: þ886 2 27376460. E-mail address: ylkuo@mail.ntust.edu.tw (Y.-L. Kuo).