From Continuous Perfectly Auto-Stirred Tank Reactor to Fixed Bed, Validation of Kinetic Laws Developed for Chemical Looping Combustion Process
Résumé
The Chemical Looping Combustion (CLC) is an oxy-combustion carbon dioxide capture process using an oxygen carrier
(NiO/NiAl2O4 in this study). The combustion is realized in two steps : The first one is the combustion of methane with nickel
oxide (called reduction step), the second one is the regeneration (reoxydation) of the reduced nickel with air (oxidation step).
The CLC process is based on this cycle of oxidation/reduction of the oxygen carrier. In order to have a better understanding of the phenomena occurring during the two steps and a better design of the oxygen carriers, the determination of the reaction kinetics during each step of the process is needed. For this purpose, some experiments and a model of a continuous perfectly auto-stirred tank reactor (CASTR) have been done and kinetic laws have been developed. In order to validate those kinetic laws, experiments in a fixed bed reactor and its modelling are also performed. The comparison of the predicted and the experimental results confirm the validity of the developed model.
Details about the kinetic laws proposed
The reactions observed in both reduction and oxidation steps of the process have been studied. The Grainy Pellet Model, using the Shrinking Core Model for reactions occurring inside the grains, were used to represent the gas-solid reactions. For example, the rate of combustion r1 of methane with nickel oxide is given by the flowing equation (1). This equation depends on the internal diffusion of gas species in a grain (De,int), the grain properties (external radius Rg and interfacial radius between Ni and NiO in a grain rg) and the tablet porosity εt.
The catalytic gas-gas reactions are described by typical Langmuir-Hinshelwood laws. For example, the rate of cracking of
methane catalysed by reduced nickel is described by equation (2). With Kj the adsorption constant of gaseous component j
and Pj its partial pressure.
A deactivation law, due to deposited carbon at the surface of the grains, is also taken into account. The kinetic laws were
developed in a CASTR and will be validated in a fixed bed reactor.
Conclusion
Experiments have illustrated the influences of operating conditions on the efficiency of the CLC process (quantity of deposited carbon, conversion of methane, …) and very good agreements are obtained between experimental and simulated results, for CASTR and fixed bed reactors. The details of kinetics and adsorption parameters used in this study will be given