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 NiAl2O4 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 NiAl2O4 remained inert during the reduction and reoxidation processes. Thermogravimetric analyses of the material under the same reducing conditions, using a 90-210 mu m 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 CH4 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: CH4 > H-2 > O-2.