Kinetic determination of a highly reactive impregnated Fe2O3/Al2O3 oxygen carrier for use in gas-fueled Chemical Looping Combustion

Abstract

The objective of this work was to determine the kinetic parameters for reduction and oxidation reactions of a highly reactive Fe-based oxygen carrier for use in chemical looping combustion (CLC) of gaseous fuels containing CH 4 , CO and/or H 2 , e.g. natural gas, syngas and PSA-off gas. The oxygen carrier was prepared by impregnation of iron on alumina. The effect of both the temperature and gas concentration was analysed in a thermogravimetric analyser (TGA).The grain model with uniform conversion in the particle and reaction in grains following the shrinking core model (SCM) was used for kinetics determination. It was 2 assumed that the reduction reactions were controlled by two different resistances: the reaction rate was controlled by chemical reaction in a first step, whereas the mechanism that controlled the reactions at higher conversion values was diffusion through the product layer around the grains. Furthermore, it was found that the reduction reaction mechanism was based on the interaction of Fe 2 O 3 with Al 2 O 3 in presence of the reacting gases to form FeAl 2 O 4 as the only stable Fe-based phase. The reaction order values found for the reducing gases were 0.25, 0.3 and 0.6 for CH 4 , H 2 and CO, respectively, and the activation energy took values of between 8 kJ mol -1 (for H 2 ) and 66 kJ mol -1 (for CH 4 ). With regard to oxidation kinetics, the reacting model assumed a reaction rate that was only controlled by chemical reaction. Values of 0.9 and 23 kJ mol -1 were found for reaction order and activation energy, respectively.Finally, the solids inventory needed in a CLC system was also estimated by considering kinetic parameters. The total solids inventory in the CLC unit took a minimum value of 150 kg MW -1 for CH 4 combustion, which is a low value when compared to those of other Fe-based materials found in the literature.

Study on Reaction Characteristics of Chemical-Looping Combustion between Maize Stalk and High Index Facet Iron Oxide

“…However, the final oxidized mass is stabilized to about 98% after several cycles. Similar to the reduction reaction mechanism model for iron based OC proposed by Cabello et al [37], the outer layer of the reduced OC is first oxidize to Fe2O3 with forming a Fe2O3 layer, and then oxygen needs to diffuse into inner layer of OC by crossing the Fe2O3 layer. On the one hand, the investigation suggests that the oxidation process is mainly controlled by diffusional effects in the product layer [40], but on the other hand the large heat released by oxidation of metallic Fe to Fe2O3 can cause serious sinter [41] preventing diffusion of oxygen and causing a small amount of the inner OC (especially close to the interface between the active component and the Al2O3 supporter) which cannot be completely oxidized.…”

“…Apparently, a faster reaction rate of Fe2O3(104)/Al2O3 can be obtained. In previous investigations [37,38], the reaction for iron based OCs reduction into Fe3O4 is very quick (just consuming a few seconds at 950 °C). Despite a slightly longer reduction time required in the present work, we should recognize the fact that reaction temperature in our work is only 800 C rather than 950 °C as used in the previous studies.…”

Studies on the redox reaction kinetics of selected, naturally occurring oxygen carrier

“…In the literature, both the shrinking core and diffusion model were the most frequently indicated for the description of the Fe oxide reduction reaction [14][15][16][17]24], which was the motivation of using both of these mentioned models in the natural oxygen carrier (Fe-based) kinetics calculations. In addition to these models, another model (e.g., the volumetric model) was also tested.…”

“…Sixty percentage of CO 2 was added to the gas mixture in the reduction reaction. The reason for using CO 2 was that carbon dioxide was an oxidation agent that helps maintain the reduction of hematite to only the magnetite phase by preventing a further reduction to wuestite or metallic Fe [24].…”

“…For both reactions, the shrinking core model was accurate. Cabello et al [24] presented results for the kinetic study of an impregnated Fe 2 O 3 /Al 2 O 3 oxygen carrier with gaseous fuels such as CH 4 , CO, and/or H 2 . The calculated reaction order values were 0.25, 0.3, and 0.6 for CH 4 , H 2 , and CO, respectively.…”