(627c) Studies On Redox Reactions of Iron-Based Chemical Looping Particles in Absence of Pore Structure
AIChE Annual Meeting
Thursday, November 11, 2010 - 1:12pm to 1:33pm
Chemical looping process, which indirectly converts carbonaceous fuel into carbon-free or carbon-negative energy through cyclic reduction-oxidation (redox) reaction of metal oxide looping particle, is considered as one of the most promising CO2 control techniques for fossil fuel conversions by the US Department of Energy. Among all the looping particles, iron based particles attract great attention since they are non-toxic and cost-efficient. During the redox reactions in a chemical looping process, iron atoms undergo cyclic valance swing among their intrinsic oxidation and/or reduction states. Since the redox cycles take place at elevated temperatures (> 750 °C), particle morphology can be affected in terms of surface area shrinking and pore structure closure. In order to better understand the performance of iron-based oxygen carriers under such reaction conditions, pure iron and iron supported by inert metal-oxide were compared from three aspects: 1) multi-cyclic reactivity; 2) morphological property; 3) ionic transfer mechanism. In the multi-cyclic redox reaction, the pure iron lost its reactivity after the first several cycles. BET tests revealed that the surface area and pore volume of pure iron particle drastically decreased after redox cycles, which can reasonably explain its deactivation. Surprisingly, the supported iron, although suffering from similar sintering effect, maintained its reactivity after 100 redox cycles. This suggests that the transport of iron cations and/or oxygen anions, as opposed to micro-and-meso pores which are nearly annihilated through sintering, may contribute to the sustained reactivity of the supported particles. Inert-maker experiments were carried out in order to identify the ionic transfer mechanism. In the case of pure iron, the oxidation process is dominated by outward diffusion of iron cations and partial inward diffusion of oxygen anions. Formation of dense iron oxide layer hinders further gas-solid interaction, and the overall reactivity depends on the outward diffusion rate of iron cations. In contrast, when the supported iron is oxidized, the dominating ionic diffusion pattern is changed to inward diffusion of oxygen anions as opposed to outward migration of iron cations. The increased ionic diffusivity of oxygen anion may explain the excellent recyclability of the supported iron. Such a change in ionic transfer mechanism may a result of the formation of oxygen vacancy.