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(¦Á-
and ¦Ã-)/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 °C). 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/¦Ã-Al2O3 |
NiO/¦Á-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 ¨C 150 |
50 ¨C 150 |
|
|
|
||
|
Figure 1: Typical mass and temperature measurement for NiO/¦Ã-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/¦Á-, ¦Ã-Al2O3 and 4% H2/Ar reaction at 800 and 950 °C. |
|
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¨C8] 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.
References:
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