Kinetic determination of chemical looping combustion reactions in a continuous stirred tank reactor : Reduction of the oxygen carrier

Abstract

This paper deals with the study of methane combustion with NiO/NiAl2O4 as oxygen carrier for a better understanding of Chemical Looping Combustion. Partial combustion of methane with NiO/NiAl2O4 tablets has been investigated in a lab scale continuous perfectly auto-stirred tank reactor (CASTR) where reacting gases methane and steam were diluted in argon and temperature ranged from 700°C to 833°C. Influence of methane and water concentrations as well as temperature are studied. Experimental results show that total oxidized compounds are firstly produced, followed by the production of hydrogen, carbon monoxide and coke, covering the particles and limiting reactions. Increasing the temperature or water concentration enable reducing the coke formation. Analysis of reaction products helps defining a reaction mechanism and ensuring absence of any mass and heat transfer limitations. A model for the reduction of the oxygen carrier, based on the grainy pellet model and taking into account the diffusion of gas species through the tablet and their reactions at the surface of each grain is developed. Gas-solid reaction rates are calculated using the shrinking core model and catalytic reaction rates are defined according to the Langmuir–Hinshelwood mechanism. The kinetic parameters of the catalytic and redox reactions involved in the oxygen carrier reduction are optimized by comparison between simulated and experimental results in the CASTR. The obtained parameters give a very good description of the observed phenomena and are in agreements with former published results.