Chemical Looping Combustion Using NiO/NiAl₂O₄:  Mechanisms and Kinetics of Reduction−Oxidation (Red-Ox) Reactions from In Situ Powder X-ray Diffraction and Thermogravimetry Experiments

In situ powder X-ray diffraction has been used to study NiO supported on NiAl 2 O 4 during several reduction−oxidation cycles, mimicking chemical looping combustion. Hydrogen and methane were used as fuel (reducing agents). Direct reduction and reoxidation of NiO/Ni is observed, and NiAl 2 O 4 remained inert during the reduction and reoxidation processes. Thermogravimetric analyses of the material under the same reducing conditions, using a 90−210 μm particle fraction suitable for fluidized-bed applications, showed that, first, a rapid reduction occurs where oxygen transport to the particle surface not is rate-limiting. The rapid reduction is followed by a much slower reduction, where oxygen transport through the particle is expected to be rate-limiting. The fast reduction reaction is determined to be first order, with respect to H 2, whereas an order slightly smaller than unity is observed when using CH 4 as a reducing agent. Reoxidation is observed to be first order, with respect to O 2 . At low reactive gas concentrations, the reaction rates decreases in the following order: CH 4 > H 2 > O 2 .