On the Kinetics of Ni-Based Oxygen Carrier Reduction and Oxidation Studied in Thermogravimetric Analysis and Fixed-Bed Reactors

On the kinetics of Ni-based oxygen carrier reduction and oxidation studied in thermogravimetric analysis and fixed-bed reactors

Zhiquan Zhou, Oscar Nordness, George M. Bollas

NiO is a promising yet expensive oxygen carrier for chemical-looping combustion (CLC). Knowledge of its inherent kinetics can greatly facilitate the optimization of the Reducer and Oxidizer in chemical-looping combustion systems. Reactivity tests with NiO/Al₂O₃(汐- and 污-)/SiO₂ oxygen carriers were performed in a thermogravimetric analyzer (see Figure 1) at atmospheric pressure to examine the supported NiO reduction kinetics by H₂ and CH₄ under isothermal conditions (600, 800 and 950 蚓). NiO/Al₂O₃/SiO₂ oxygen carriers were prepared via the incipient wetness impregnation method. Table 1 shows the main properties of the fresh (before CLC) materials.

Table 1: Properties of the fresh NiO-based oxygen carriers

Properties	NiO/污-Al2O3	NiO/汐-Al2O3
BET surface area (m2/g)	94	27
Pore size (Å)	116	294
Total NiO content (%)	32	24
Active NiO content (%)	25	21
XRD phases	NiO, NiAl2O4, Al2O3, SiO2	NiO, NiAl2O4, Al2O3, SiO2
Apparent density (g/cm3)	1195	1414
Particle size (µm)	50 每 150	50 每 150

 

Figure 1: Typical mass and temperature measurement for NiO/污-Al2O3 oxygen carrier and 4% H2/Ar for reduction and air for oxidation reactions.		Figure 2: Effect of reaction temperature and Curve fitting of Avrami-Erofe'ev and shrinking core models to reduction conversion for NiO/汐-, 污-Al2O3 and 4% H2/Ar reaction at 800 and 950 蚓.

            In this presentation, model-fitting (conversion (X) vs. time and dXdt vs. conversion) and model-free methods (Hancock and Sharp [1]) are used to evaluate the kinetics mechanism of the reduction and oxidation processes. Over 20 solid-state reaction models (nucleation, geometrical contraction, diffusion, reaction-order, etc.) [2,3] are applied and compared against reduction and oxidation experiments of bulk Ni and supported Ni carriers reported in the  literature [4每8] and generated in this work (Figure 2). The F-test and Akaike Information Criterion [9] are used to statistically compare models with different independent variables. Consistent kinetic mechanisms are derived that are capable of describing the reactivity performance of different Ni-based oxygen carriers. 

Acknowledgement: This material is based upon work supported by the National Science Foundation under Grant No. 1054718.

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