Kinetic modeling of NiO-based oxygen carriers for the sorption enhanced chemical looping steam CH4 reforming
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2018 Elsevier Ltd. The reduction and oxidation (redox) kinetics of four NiO-based oxygen carriers (OCs) under chemical looping conditions are presented and discussed in this study. The 40% NiO OCs were supported on Al 2 O 3 , TiO 2 , SiO 2 and ZrO 2 and were isothermally tested in a thermogravimetric unit connected with a mass spectrometer (TGA-MS) in 20 redox cycles (reduction under 15% CH 4 /He at 650C/oxidation under air at 800C). Several solid-state kinetic models (chemical reaction controlled, geometrical/volumetric contraction, diffusion limited, nucleation growth and random pore models) were consecutively and optimally screened for each OC and for different redox cycles. As was found, metal-support interactions occurring between NiO and Al 2 O 3 , TiO 2 supports lead to a reduction controlled by chemical reaction and was quantitatively captured through the "Unreacted Shrinking Core Model". On the contrary, reduction of NiO supported on SiO 2 , TiO 2 is controlled by "nucleation and nuclei growth" and Avrami-Erofeev kinetics fitted the respective experimental data. Regarding oxidation, the present study revealed that it is controlled by nucleation and "Avrami-Erofeev" (NiO/Al 2 O 3 , NiO/TiO 2 ) and "Prout-Tompkins" (NiO/ZrO 2 ) kinetic models provided an optimal fitting. In all studied cases, the progressing of cycles revealed structural material modification that were summed up to significant variations in OCs' redox rates and related kinetics.
