Enhanced performances of iron-rich sludge ash for chemical looping gasification of pretreated rice straw by the modification of transition metal oxides

• Construction of bimetallic oxygen carriers significantly enhanced reaction performance. • Ni doping exhibited the best performance with notable structural and reactivity improvements. • XPS, H 2 -TPR and SEM-EDS analyses revealed the structure–performance relationship. • Optimized gasification parameters were identified, leading to markedly improved efficiency. • The 10 % NiO-doped sample showed excellent redox stability over multiple cycles. This study aimed to develop efficient waste-derived oxygen carriers (OCs) for biomass chemical looping gasification (BCLG) by modifying iron-rich sludge ash (SA) with transition metal oxides (NiO, CuO, and CoO) through a water-bath co-precipitation method. The structural and redox properties of the modified OCs were systematically characterized. The results indicate that metal doping significantly enhanced the redox capacity and reactivity of the OCs, with NiO-modified samples exhibiting superior reduction performance. Nevertheless, excessive doping (30 wt%) induced surface particle agglomeration, leading to reduced activity. BCLG experiments were conducted using acetic acid-washed and torrefied straw at 260 °C (TWRS260) as the fuel. The influences of reaction temperature and biomass-to-OCs mass ratio on gasification performance were investigated. Ni-based OCs demonstrated higher reactivity and selectivity. As the temperature increased from 700 °C to 900 °C, the carbon conversion efficiency ( η C ) and cold gas efficiency ( CGE ) improved significantly, rising from 42.89 % and 44.47 % to 84.48 % and 89.49 %, respectively, with optimal performance observed at 800 °C. At a biomass-to-OCs mass ratio of 1:1, η C and CGE reached 69.41 % and 76.24 %, respectively. Moreover, the 10 wt% NiO-doped SA exhibited excellent initial reactivity and cyclic stability over multiple redox cycles, outperforming the unmodified counterpart. These findings not only advance the understanding of waste-based OCs in BCLG but also highlight their potential for sustainable energy conversion and carbon–neutral biomass utilization.