Hydrogen-rich syngas production from straw char by chemical looping gasification: The synergistic effect of Mn and Fe on Ni-based spinel structure as oxygen carrier

•NiMn2O4 oxygen carrier shows great perform for hydrogen-rich syngas production at 800 °C.•NiMn2O4 and NiFe2O4 oxygen carriers shows different reduce pathway for Ni formatted.•The NiMn2O4 and NiFe2O4 oxygen carriers compares with reactivity.•The conditions for hydrogen-rich syngas production were considered at temperature, steam flow and ratio of oxygen carrier with straw char. Chemical looping gasification (CLG) is one of the promising ways to exploit biomass resources and produce hydrogen-rich syngas. In this work, two spinel oxygen carrier (NiMn2O4 and NiFe2O4) were studied for hydrogen-rich syngas production. A series of characterizations techniques including XRD, XPS, BET, SEM, TPR-H2 and TPD-CO2 were used to investigate the fresh and spent oxygen carriers. The result reveals that the NiMn2O4 oxygen carrier has higher reactivity and redox performance due to its higher oxygen vacancy concentration. On the other hand, the NiMn2O4 oxygen carrier obtain greater reactivity for hydrogen generation due to its phases and structure changes. NiMn2O4 oxygen carrier was reduced form (NiO)0.25(MnO)0.75 to (NiO)0.75(MnO)0.25 to Ni + MnO phases at 650-850 °C. Ni generated adhere to sufure of MnO, and the structure become loose and porous. And the more oxygen vacancise was formed. However, NiFe2O4 oxygen carrier showed different reduction behavior, it reduced from Ni1.25Fe1.85O4 to Fe2O3 + Ni3Fe phases at 650–850 °C. The formed Ni phase for NiMn2O4 oxygen carrier promote hydrogen gas generation (2.65Nm3/kg) at 800 °C. Based on the charaterize analysis, the proposed reaction process was explained. For the hydrogen-rich syngas production, the optimum conditions in this works shows at temperature (800 °C), steam flow (80 μL/min) and ratio of oxygen carrier with straw char (1:1).