Lattice oxygen activity regulation by alkaline earth metals in iron oxides for biomass chemical looping gasification

Abstract

Biomass chemical looping gasification (BCLG) represents a highly promising approach for syngas production. A critical factor in BCLG is the selection of suitable oxygen carriers (OCs) that exhibit both high carbon conversion (η_C) and CO selectivity (S_CO). In this study, iron-based OCs were modified with various alkaline earth metals (AEMs, i.e. Ca, Sr, and Ba) to modulate lattice oxygen activity. The effects of oxygen-to-carbon ratio (O/C), temperature, and cyclic operation on BCLG performance were investigated in a fixed-bed reactor. Among the AEM-modified OCs, Ca1Fe2 (spinel), Sr1Fe1 (perovskite), and Ba1Fe2 (spinel), showed superior performance compared to their Ca, Sr, and Ba-Fe counterparts, respectively. At 900 °C and O/C = 2, the pristine Fe₂O₃ exhibited a η_C of 82 % and S_CO of 53 %. The η_C for Sr1Fe1 and Ba1Fe2 reached >90 % at 900 °C, with S_CO increased to >70 %, resulting in a significantly higher syngas yield (H₂+CO) of >800 mL/g-biomass (vs. 560 for Fe₂O₃). In contrast, the addition of Ca showed a much less pronounced effect. In the cyclic test at 900 °C, Ba1Fe2 showed the poorest stability due to a severe sintering. Sr1Fe1 presented the best stability with a syngas yield of 722 mL/g after 10 cycles. The decrease in the activity of Sr1Fe1 was mainly due to the phase separation of SrFeO_3-x after multiple cycles. Thermodynamically, Sr1Fe1 is favorable for the production of CO instead of CO₂, leading to its intrinsic high selectivity. As demonstrated by ¹⁸O-isotopic exchange and H₂-TPR, the activity of surface lattice oxygen and the diffusivity of bulk lattice oxygen was boosted by Sr addition, which caused the high η_C and S_CO of Sr1Fe1 at the same time. Thus, even at O/C=5, S_CO for Sr1Fe1 reached 64 % with η_C up to 99 %, comparing to the S_CO=31 % and η_C=91 % for Fe₂O₃.