Reduction Kinetics Of Fe₂O₃ For Chemical Looping Hydrogen Generation

Chemical looping hydrogen generation（CLHG）is developed from both chemical looping combustion（CLC）and hydrogen production by the steam-iron process.CLHG produces high-purity hydrogen while achieving inherent CO₂ separation.The deep reduction for CLHG is proposed to improve the efficiency of hydrogen production by operating multiple packed beds in series in reduction stage.In this study,the reduction behaviors and kinetics of Fe₂O₃ was investigated by thermogravimetric analysis（TGA）to predict the reduction degree and reaction rate in two packed bed reactors.The continuous reduction process of Fe₂O₃ can be mainly described as a two-step reduction（Fe₂O₃→FeO and FeO→Fe）.The reduction rate increased with temperature,the hydrogen content in CO–H₂ mixed gas,and reducing composition in inert gas.For Fe₂O₃→FeO,the reaction mechanism can be described by nucleation and growth model.The activation energies under CO:H₂=1:0,CO:H₂=1:0.33,CO:H₂=1:0.5,CO:H₂=1:1 and CO:H₂=0:1were estimated to be 24.10,27.86,29.30,31.83 and 36.74 kJ/mol,respectively.For FeO→Fe,the reaction mechanism of the first half can be described by bi-dimensional phase boundary controlled reaction at any content ratio of reducing composition.The last stage of FeO→Fe was controlled by diffusion reaction.Meanwhile,The increase of content ratios of inert composition did not change the value of activation energy.The CO/CO₂ mixtures was used as reducing gases to investigate the reduction behaviors and kinetics parameters of iron-based oxygen carrier in the second reactor.Based on the thermodynamics properties of the Fe-CO-CO₂ system,CO/CO₂ mixtures in different volume ratios can decouple the reduction of Fe₂O₃ into three relatively independent steps（Fe₂O₃→Fe₃O₄,Fe₃O₄→FeO and FeO→Fe）.The reaction of step 1 and 2 are phase boundary reaction;the step 3 is controlled by nucleation and nuclei growth model and diffusion model.The activation energy of the three steps including Fe₂O₃→Fe₃O₄,Fe₃O₄→FeO and FeO→Fe is estimated to be 34.92±1.24,70.13±0.88 and 44.12±1.44 kJ/mol,respectively.The Fe₃O₄→FeO have the highest activation energy with a maximum reaction rate,and FeO→Fe has slowest reaction rate with 2/3 lattice oxygen consumption theoretically.From the point of operation of double-tower,as for the first reactor,the first reactor should stop the reduction reaction before that the iron-based oxygen carrier was controlled by diffusion model.As for the second reactor,the high CO₂ content will restrict the phase of iron-based oxygen carriers to stay in Fe₃O₄ or FeO.So the necessity of hydrogen generation for the second reactor is limited.