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

Abstract

In situ powder X-ray diffraction has been used to study NiO supported on NiAl 2 O 4 during several reductionoxidation 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 ratelimiting. 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 .

Progress in Chemical-Looping Combustion and Reforming technologies

“…This case is characterized by a straight conversion vs. time curve. Straight conversion vs. time curves also has been obtained for particles for which the plate-like geometry can not be assumed [243,377]. The linearity of the curves may indicate that the oxygen coverage on the surface is constant during the reducing period, meaning that oxygen atoms diffuse through the product layer of metal onto the surface where the reaction occurs [377].…”

Kinetic determination of a highly reactive impregnated Fe2O3/Al2O3 oxygen carrier for use in gas-fueled Chemical Looping Combustion

“…with the Fe20Al oxygen carrier. The reacting model for reduction kinetics assumes a first step with the reaction rate controlled by chemical reaction in the grain surface [53][54]. During this first step, alumina and oxygen must diffuse towards the reaction interphase.…”

“…Moreover, the reduction rate during this step is independent of the fuel gas concentration. This fact suggests that diffusion of the reacting gas is blocked, but oxygen must diffuse outwards [53]. Taking the above comments and conditions in the TGA into consideration, the dependence of solids conversion with time for this Fe-based oxygen carrier is described by the following equation [55]:…”

Advancements in Development of Chemical-Looping Combustion: A Review

“…Another advantage of using Al 2 O 3 as inert binder is that it is much cheaper than YSZ powder. Therefore, Al 2 O 3 has attracted wide attention due to its favorable fluidization properties thermal stability, and low cost [15][16][17][18]. It is noted that a part of NiO is converted to form metal aluminum spinel compounds, NiAl 2 O 4 , via solid-state reaction with Al 2 O 3 in the course of the sintering process [19].…”