Design and evaluation of Fe-based oxygen carriers for biochar chemical looping gasification

Iron (Fe)-based oxygen carriers (OCs) have been widely investigated for chemical looping gasification (CLG); however, most previous studies have focused on steam-assisted conversion of raw biomass in fluidised-bed systems, where devolatilisation, tar formation, char gasification, and OC redox cycling occur simultaneously, complicating interpretation of individual reaction steps. In particular, CO 2 char gasification, a key rate-limiting reaction governing syngas composition and carbon conversion efficiency (CCE), remains insufficiently understood for Fe-based OC systems. In this study, biochar is deliberately employed as a model carbon feedstock to decouple char gasification from biomass pyrolysis and tar-related phenomena, enabling focused investigation of OC-carbon interactions under CO 2 -rich fixed-bed chemical looping conditions. The effects of operating temperature (700–900 °C), oxygen carrier-to-biochar (OC:BC) mass ratio (1:1–20:1), and metal doping of Fe-based OCs with cerium (Ce), cobalt (Co), nickel (Ni), lanthanum (La), and strontium (Sr) were evaluated under CO 2 gasification conditions, using a fixed Fe-Al-Me (70:20:5) formulation to enable orderly comparison of dopant effects under reaction-limited conditions. The results demonstrate that temperature strongly influences gasification performance, with limited CCEs of approximately 10 wt% at 700 °C for both biochar (BC) and Fe 2 O 3 -biochar (Fe-BC) mixtures. Substantial improvements were observed at higher temperatures, with CCEs of 33.8 wt% and 50.4 wt% obtained for BC and Fe-BC at 800 °C, respectively, while further increases to 57.0 wt% and 94.0 wt% were achieved at 900 °C. Optimal gasification performance exceeding 90 wt% CCE was obtained at OC:BC of 5:1 and 10:1 at 900 °C over 120-minutes. At 800 °C, Al 2 O 3 -supported Fe 2 O 3 exhibited lower gasification activity (46.4 wt% CCE) than unsupported Fe 2 O 3 (54.7 wt% CCE), but showed enhanced structural stability as evidenced by post-reaction characterisation. La modification significantly enhanced Fe-Al OC performance, increasing CCE from 46.4 wt% to 62.8 wt%, whereas Ni doping produced no substantial improvement compared to undoped Fe-Al. XPS analysis indicated that dopant addition modified the surface oxygen environment without fundamentally altering the Fe 2 O 3 -based redox chemistry, suggesting that differences in gasification performance were associated with oxygen exchange behaviour rather than changes in the dominant Fe redox pathway. Short-term three-cycle testing of Fe-Al-La indicated retention of gasification activity following successive gasification-reoxidation treatments, demonstrating moderate stability under the conditions investigated.