Behavior of ilmenite as oxygen carrier in chemical-looping combustion A. Cuadrat, A. Abad ⁎, J. Adánez, L.F. de Diego, F. García-Labiano, P. Gayán Instituto de Carboquímica (C.S.I.C.), Dept. of Energy & Environment, Miguel Luesma Castán, 4, Zaragoza, 50018, Spain abstract article info Article history: Received 7 June 2011 Received in revised form 24 October 2011 Accepted 24 October 2011 Available online 29 November 2011 Keywords: CO 2 capture Chemical-looping combustion Oxygen-carrier Ilmenite Fuel For a future scenery where will exist limitation for CO 2 emissions, chemical-looping combustion (CLC) has been identified as a promising technology to reduce the cost related to CO 2 capture from power plants. In CLC a solid oxygen-carrier transfers oxygen from the air to the fuel in a cyclic manner, avoiding direct contact between them. CO 2 is inherently obtained in a separate stream. For this process the oxygen-carrier circulates between two interconnected fluidized-bed reactors. To adapt CLC for solid fuels the oxygen-carrier reacts with the gas proceeding from the solid fuel gasification, which is carried out right in the fuel-reactor. Ilmenite, a natural mineral composed of FeTiO 3 , is a low cost and promising material for its use on a large scale in CLC. The aim of this study is to analyze the behavior of ilmenite as oxygen-carrier in CLC. Particular attention was put on the variation of chemical and physical characteristics of ilmenite particles during consecutive redox cycles in a batch fluidized-bed reactor using CH 4 ,H 2 and CO as reducing gases. Reaction with H 2 was faster than with CO, and near full H 2 conversion was obtained in the fluidized-bed. Lower reactivity was found for CH 4 . Ilmenite increased its reactivity with the number of cycles, especially for CH 4 . The structural changes of ilmenite, as well as the variations in its behavior with a high number of cycles were also evaluated with a 100 cycle test using a CO + H 2 syngas mixture. Tests with different H 2 :CO ratios were also made in order to see the reciprocal influence of both reducing gases and it turned out that the reaction rate is the sum of the individual reaction rates of H 2 and CO. The oxidation reaction of ilmenite was also investigated. An activation process for the oxidation reaction was observed and two steps for the reaction development were differenced. The oxidation reaction was fast and complete oxidation could be reached after every cycle. Low attrition values were found and no defluidization was observed during fluidized-bed operation. During activation process, the porosity of particles increased from low porosity values up to values of 27.5%. The appearance of an external shell in the particle was observed, which is Fe enriched. The segregation of Fe from TiO 2 causes that the oxygen transport capacity, R OC , decreases from the initial R OC =4.0% to 2.1% after 100 redox cycles. © 2011 Elsevier B.V. All rights reserved. 1. Introduction At present there is a general assent on the need of reducing the emissions of the greenhouse gas CO 2 in order to restrain climate change. Anthropogenic CO 2 is mainly generated in combustion of fossil fuels, which are foreseen to provide about 80% of the overall world consumption of energy for the next several decades. For the power generation and heat supply sector, emissions were 12.7 Gt CO 2 -eq in 2004, which is 26% of total CO 2 -eq emissions. When regarding the energy-related CO 2 emissions by fuel type, coal use generated 39% of the emissions in 2004 and it is estimated that the percentage in 2030 will rise up to 43% [1,2]. Among the different opportunities to reduce the anthropogenic CO 2 emissions, the development of tech- nologies to capture CO 2 from fossil fuel uses and to store it perma- nently has been identified as a relevant option in the future, being the implementation of these technologies more feasible and readily in stationary power plants. In this context, chemical-looping combustion (CLC) is one of the most promising technologies to carry out the CO 2 capture at a low cost [3–5]. CLC is based on the transfer of the oxygen from air to the fuel by means of a solid oxygen-carrier that circulates between two interconnected fluidized-beds: the fuel- and the air-reactor [6]. In the fuel-reactor the oxygen-carrier is reduced through oxidation of the fuel. Afterwards the oxygen-carrier is directed to the air-reactor, where it is again regenerated, as the inlet air flow reacts with the solid. The net chemical reaction is the same as at usual combustion with the same combustion heat released. Important progress has been made in CLC with natural gas to date. Several authors have successfully demonstrated the feasibility of this process in different CLC prototypes in the 10–140 kW th range using oxygen-carriers based on NiO [7–10] and CuO [11]. But increasing interest is found about the application of CLC using coal as fuel, regarding the intensive use of this fuel. There are two possibilities for the use of the CLC technology with coal. The first one is to carry out previous coal gasification and subsequently to Fuel Processing Technology 94 (2012) 101–112 ⁎ Corresponding author. E-mail address: abad@icb.csic.es (A. Abad). 0378-3820/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2011.10.020 Contents lists available at SciVerse ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc