Chemical looping gasification coupled with steam reforming of biomass using NiFe2O4: Kinetic analysis of DAEM-TI, thermodynamic simulation of OC redox, and a loop test

Abstract

NiFe2O4 is one of the best oxygen carriers (OCs) for the biomass chemical looping gasification coupled with steam reforming. The kinetics analysis of biomass and NiFe2O4 is helpful for understanding their mechanism of interaction. In this work, a thermogravimetric analyzer, and distributed activation energy model based on a time integral (DAEM-TI) were used to study the effect of the OC/biomass ratio and atmosphere on gasification. In addition, thermodynamic analysis by Factsage and a 15-loop test of the OC in a fixed bed reactor were applied to investigate the NiFe2O4 transformation and its multiple redox performance. The kinetic analysis showed that gasification included: i) moisture loss of ~100 kJ/mol, ii) devolatilization of 150 ~ 175 kJ/mol, and iii) char gasification of 250 ~ 350 kJ/mol. There was a transition that was accompanied by a slight reduction before a rapid increment of activation energy in adjacent stages. This may be attributed to the difference between the various energy barriers of reactions. The simulation results of DAEM-TI matched the experimental data well, and all adjusted coefficients of determination exceeded 98%. The best ratio of OC/biomass was 5/5 for optimal gasification that avoided overoxidation. The mass rebound of H2O on char gasification was higher than that of CO2, which indicated H2O had stronger oxidizability than CO2. Thermodynamic simulation showed that spinel NiFe2O4 converted into an FCC Fe(Ni) alloy during gasification. However, CO2 and H2O were only capable of restoration to Fe3O4 and could not oxidize Ni to Ni2+, so calcining in air was necessary for NiFe2O4 restoration. The performance of restored NiFe2O4 remained high and even slightly improved compared to that of the fresh NiFe2O4 after 15 redox loops.