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Determination and Comparison of CuO Reduction/Oxidation Kinetics in CLC Experiments with CO/Air by the Shrinking Core Model and Its Characterization

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journal contribution
posted on 17.12.2015, 02:07 by Chiranjib Saha, Sankar Bhattacharya
The Shrinking Core Model (SCM) has been applied to determine and experimentally validate the reduction and reoxidation (redox) reaction kinetics of CuO in an in situ chemical looping combustion (CLC) process. This paper focuses on the determination of redox kinetics of CuO with one of the major coal gasification products i.e. carbon monoxide (CO) and air in a CLC process using a thermogravimetric analyzer (TGA). The comparison of the kinetic parameters of CuO obtained in CLC experiments, using CO and air as reducing and oxidizing atmosphere, respectively, with the predictions by the SCM model are also presented in this paper. The CuO particles are characterized in detail to obtain structural and elemental changes, due to their cyclic use in CLC experiments with CO/air, compared to the fresh particles. It has been observed that the reduction reaction control mechanism of SCM predicts very well the conversions of CuO during reduction in CO. However, the reoxidation of reduced CuO particles are governed by product layer diffusion controlled mechanism. Based on the kinetics data obtained from experiments, two generalized equations are formed for determination of reaction rate constant and effective diffusivity. A sensitivity analysis shows that both the reduction and reoxidation reactions respond equally to self-stabilize the system if the kinetic parameters are disturbed by ±5%. The CuO particles are characterized by scanning electron microscopy (SEM), energy dispersive X-rays (EDX), and X-rays photoelectron spectroscopy (XPS) to support the experimental results. A good agreement between predictions and the experimental values are achieved for all the cases studied. The maximum error percentage between predictions and experiments range within ∼(0.5–2)%. It is also interesting to notice that as the particle size increases, the reduction kinetic parameter error percentage increases, but (re)­oxidation error percentage decreases. This supports the kinetic result that describes the reduction to be chemical reaction controlled and (re)­oxidation to be diffusion controlled as an increase in the particle size generally moves the reactions toward diffusion controlled regime. Also the order of reaction, determined in this part, supports the assumption for application of SCM in this particular case. It is experimentally observed, by different solid characterization techniques, that the core of the CuO particles remain unreacted during a CLC process. Overall, it can be concluded that the shrinking core model is applicable to a CLC process. However, experimental validation with other oxygen carriers may be required.