Redox kinetics of NiO/YSZ for chemical-looping combustion and the effect of support on reducibility

This paper explores the reaction kinetics of NiO supported on YSZ (Yttria Stabilized Zirconia) as an oxygen carrier for chemical looping combustion. Nickel particles with size less than 1 mu m mixed with YSZ nano-powders are used to prepare the solid mixture, with 45% mol of NiO. Redox reactivity and oxygen car-rying capacity are measured in a laboratory scale fixed bed reactor in the temperature range 500-1000 degrees C with different concentrations of the reactive gasses. Samples are subjected to repeated redox cycles using synthetic air (O2 + Ar) for oxidation, and H2/H2O/Ar mixtures for reduction. NiO/YSZ demonstrates superb cyclic re-generability starting with the 2nd cycle, with full utilization of its oxygen carrying capacity. Compared to pure nickel, pronounced improvement is achieved in the kinetics and oxygen utilization. Full reduction is achieved, and the presence of H2O does not affect the reduction rate. Reactivity is also determined as a function of conversion. Global models of redox conversion are developed, in which surface chemistry and solid diffu-sion are considered. Oxidation exhibits the characteristics of a shrinking-core model with internal reactions at the Ni/NiO interface being the rate limiting step, and it is weakly temperature dependent. Reduction with H2 generally exhibits surface chemistry limitation (adsorption-desorption), with surface product formation being the rate limiting step. YSZ significantly enhances ionic transport during oxidation and reduction. Re-action rate dependencies on conversion during the two steps suggest an optimal range for the oxygen carrying capacity of the material.(c) 2023 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

This paper investigates the reaction kinetics of NiO supported on YSZ as an oxygen carrier for chemical looping combustion. The study examines the reactivity and oxygen carrying capacity of NiO/YSZ in a fixed bed reactor under different temperatures and concentrations of reactive gases. The results demonstrate that NiO/YSZ exhibits excellent cyclic regenerability and improved kinetics and oxygen utilization compared to pure nickel. The study also develops global models for oxidation and reduction processes, highlighting the effects of surface chemistry and solid diffusion.