Kinetic analysis of biochar chemical looping gasification with calcium ferrite as oxygen carriers

The chemical looping gasification (CLG) kinetics of biochars with calcium ferrite as oxygen carriers and the effects of different kinds of calcium ferrite and biochars were investigated by TGA. The properties of biochars and calcium ferrite were analyzed by XRD, SEM, BET, etc. The Škvára-Šesták method was used to determine the kinetic mechanism function. The results show that the reduction reaction rate and the oxygen carrying capacity of oxygen carriers follow the sequence: Ca2Fe2O5 > CaFe2O4 > Fe2O3, and CaFe2O4 > Ca2Fe2O5 > Fe2O3, respectively. The oxygen carriers can be completely reduced to Fe and CaO by biochar. The activation energy of CaFe2O4 reduction is in the range of 167.44–600.83 kJ/mol; and the activation energy of Ca2Fe2O5 reduction is in the range of 413.62–583.51 kJ/mol. The CaFe3O5 generated during the reduction of CaFe2O4 may have a negative influence on the lattice oxygen diffusion. The reduction of CaFe2O4 can be divided into two stages: when the conversion degree α is less than 0.15, the CaFe2O4 is reduced to Ca2Fe2O5 following the random nucleation and nuclei growth model; when α is greater than 0.15, Ca2Fe2O5 is further reduced to CaO and Fe following the 3-D diffusion mechanism. The mechanism function of the reduction of Ca2Fe2O5 is the same as that of the second stage of CaFe2O4 reduction.