Performance of Fe–Ni bimetallic oxygen carriers for chemical looping gasification of biomass in a 10 kWth interconnected circulating fluidized bed reactor

Abstract

Chemical-looping gasification (CLG) of biomass was investigated in a 10 kWth interconnected fluidized-bed reactor with Fe–Ni bimetallic oxides as oxygen carriers (OC). The thermodynamic analysis of the CLG indicated that the temperature range of 700 °C–950 °C can be suitable for the CLG reactions. The fluidized bed experimental results indicated that biomass was partially oxidized to synthesis gas by the lattice oxygen of the oxygen carrier in the fuel reactor and the synthesis gas composition of CO, H2 and CH4 as well as carbon conversion rate and gasification efficiency increased with the rising reaction temperature, while the CO2 fractions decreased due to the exothermic process. Carbon conversion decreased with the rising ratio of biomass to oxygen carrier. However, there was an optimal value of the gasification efficiency at 70.48% corresponding to the feeding rate of 1.6 kg/h. Compared to the Fe2O3/Al2O3 oxygen carrier, the Fe–Ni bimetallic oxygen carriers displayed a higher the gasification efficiency of biomass, which was correlated with the increasing composition of CO, H2 caused by the synergistic effect between Fe2O3 and NiO. The fresh and used oxygen carriers were characterized by means of XRD, SEM/EDX and BET. The XRD results indicated the mainly reduction products of the oxygen carrier were Fe3O4 in the CLG of biomass and new phase of Ni–Fe kamacite was formed during redox cycles, which could improve the reactivity of the oxygen carrier. Also, the oxygen carriers can be well regenerated and kept good crystalline state. The surface area of the oxygen carrier decreased from 2.486 m2/g to 2.085 m2/g after 120min CLG reactions. The average pore diameter and total pore volume shifted from 39.8 nm, 0.0248 cc/g to 39.4 nm, 0.0205 cc/g, respectively, suggesting that a part of micorpores in the particles of the oxygen carrier were blocked or collapsed. However, reactivity deterioration of the Fe–Ni bimetallic oxygen carrier was not observed during the experiment operation. SEM-EDX results showed that the metal elements were distributed uniformly in the fresh and used oxygen carriers and irregularly blocky particles with smaller average size and porous structure were still present in regenerated oxygen carrier samples. Overall, these results suggested that the Fe–Ni bimetallic oxygen carrier was a good candidate for the CLG of biomass.