Description of the Aging Phenomenon of Oxygen Cariier Particles in the Chemical Looping Combustion Process
Résumé
The process of Chemical Looping Combustion necessitates the use of two reactors. In the first one, called “fuel
reactor”, the reaction of combustion of methane with an oxygen carrier takes place (reduction step of the oxygen
carrier), then, the oxygen carrier is re-oxidised with air in the second reactor called “oxidation reactor” (oxidation step). Diminution of the oxygen carrier performances after some cycles of reduction and oxidation are observed. The main objective of this study is to understand the phenomena leading to a decrease of the process performances. For this purpose, some experiments and modelling of fixed bed reactor have been performed to determine the relation between oxygen carrier structure changes (variation of porosity, chemical composition, structural organisation,…) and process performances decrease (variation of methane conversion and oxygen carrier regeneration rate, …). In order to qualify the variation of physical properties of the oxygen carrier material, analysis like X-ray diffraction, BET surface measurements and scanning electron microscopy (SEM) observations, are done to bring out the variation of structure of oxygen carrier particles. The model of the used fixed bed is modified to take into account all the phenomena observed.
Several points will be presented concerning the degradation of the oxygen carrier particles:
- even in case of fixed bed reactor (avoiding high friction between particles), the shape of the oxygen carrier grains may
varies after several cycles of oxidation/reduction of the material. BET analysis will be done to quantify the impact of those
modifications on the reacting surface of each grain,
- X-ray diffraction analyses will show the evolution of the proportion of NiO, Ni and NiAl2O4 in the oxygen carrier grains. The
analysis of the X-ray signals should indicate that there are some structural modifications of the oxygen carrier.
- after the reoxidation of the oxygen carrier, the grains are stuck to each other and form aggregates, measurements will be
done to determine if these aggregates impact the diffusion of gas species through the grains,
- a complete analysis of the impact of experimental conditions on the structure of the oxygen carrier will be presented,
- carbon deposition has a strong impact on the performances of the oxygen carrier.
Conclusion
The modification of the oxygen carrier structure and composition has an impact on its performances in the CLC process
(variation of methane conversion, …). A model was developed to represent one cycle of oxidation/reduction of the oxygen carrier in a previous study. The description of the modifications of the material will be used to extend the validity of this model to several cycles. The impact of different experimental conditions on the process performances is also studied.