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

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 (ηC) and CO selectivity (SCO). 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 ηC of 82 % and SCO of 53 %. The ηC for Sr1Fe1 and Ba1Fe2 reached >90 % at 900 °C, with SCO 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 SrFeO3-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 ηC and SCO of Sr1Fe1 at the same time. Thus, even at O/C=5, SCO for Sr1Fe1 reached 64 % with ηC up to 99 %, comparing to the SCO=31 % and ηC=91 % for Fe2O3.