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.