Behavior of ilmenite as oxygen carrier in chemical-looping combustion

For a future scenery where will exist limitation for CO2 emissions, chemical-looping combustion (CLC) has been identified as a promising technology to reduce the cost related to CO2 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. CO2 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 FeTiO3, 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 CH4, 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 CH4. Ilmenite increased its reactivity with the number of cycles, especially for CH4. 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 TiO2 causes that the oxygen transport capacity. R-OC, decreases from the initial R-OC = 4.0% to 2.1% after 100 redox cycles. (C) 2011 Elsevier B.V. All rights reserved.