Kinetics of NiO reduction by H2 and Ni oxidation at conditions relevant to chemical-looping combustion and reforming
Graphical abstract
Introduction
Research has shown that nickel oxide (NiO) is a promising oxygen carrier (OC) for chemical-looping combustion (CLC) [1]. Therefore, understanding of the intrinsic kinetics of NiO reduction is crucial for designing efficient CLC systems. Although a breadth of literature exists on the gas–solid kinetics of NiO reduction and Ni oxidation, most studies are not applicable to CLC conditions. The published kinetics is limited to the use of fresh or pre-treated particles, while in the CLC environment the solid particles undergo repeated redox cycles. Moreover, the experimental conditions, e.g., temperature, particle size and gas composition, are usually different than those required for CLC [2]. Finally, the kinetics of NiO reduction and Ni oxidation is often derived from free bulk NiO; whereas, in CLC NiO is dispersed on a support, which increases the active surface area and mechanical strength of the oxygen carrier. Thus, limited information can be extracted from the available reactivity data to determine NiO/Ni reactivity with certainty at CLC-relevant conditions, which is a necessary prerequisite for design purposes.
Generally, the detailed kinetic mechanisms involve a group of individual chemical steps, by which a reactant is converted into a product. However, the information on chemical steps is usually difficult to obtain. Therefore, in solid-state kinetics, the kinetic mechanism study usually involves application and verification of a reasonable model to the experimental data. Ideally, the selection of a best-suited solid-state reaction model is supported by other complementary techniques, such as microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), etc. [3]. Mechanisms for solid-state reactions include reaction-order models (F), geometrical contraction models (R), diffusion-limited models (D), nucleation models (Avrami–Erofe'ev, AE) [4], [5], [6], [7], the random pore model (RPM) [8], and the Šesták–Berggren (SB) [9] and Prout–Tompkins (PT) [10] models. Table 1 presents all these models, as they have been adopted in this work, along with their naming and formulas. The principal assumptions and physical meaning of these models are discussed briefly in Section ‘Solid-state kinetic model and mechanisms’.
Table 2 summarizes the relevant kinetic studies on NiO reduction and Ni oxidation in the literature. The most significant observations are discussed here to illustrate the lack of consistency in previous analyses. Janković et al. [11], [12], [13], [14] used the SB model to describe the reduction of NiO in a thermogravimetric analyzer (TGA). Richardson et al. [15], [16] used in-situ hot-stage XRD and observed that the R3 model cannot describe the reduction of NiO. By examining NiO powders in an environmental transmission electron microscope (ETEM) in the presence of H2, Jeangros et al. [17] proposed that the AE model with the rate of nuclei growth as the rate determining step describes the experimental data accurately. Plascencia and Utigard [18] concluded that reduction kinetics of Sinter 75 and Tokyo NiO highly depends on the morphological features and the R3 model is not suitable, based on SEM observations. Instead, they proposed a mixed controlling mechanism with the R and D models as more suitable. Optical microscopy observations led to the conclusion by Utigard et al. [19] that the R3 model represents the kinetics of the reduction of Goro NiO. By comparing the differences in the reduction behavior of bulk NiO and silica-supported NiO, Syed-Hassan and Li [20], [21] related the incubation period of the Ni reduction with the formation of radical species. Lee and Kim [22] studied the kinetics of H2 reduction of nanocrystalline NiO, concluding to the dominance of reaction control at lower temperatures, whereas diffusion control dominates at higher temperatures, which was postulated to be the result of structural changes with temperature [22]. Erri and Varma [23] published a kinetic study of NiO and NiO/NiAl2O4 and concluded that the inconsistencies in the literature value of the activation energy are due to a lack of understanding and consideration of diffusional effects.
Extensive studies on the reduction kinetics of supported NiO have also been published (Table 2), among which studies on Al2O3 (α- and γ-Al2O3) are prominent. Dueso et al. [24], [25] selected the R3 and D4 models for Ni reduction and oxidation kinetics of NiO and NiAl2O4 supported on Al2O3. Ryu et al. [26], [27], [28] investigated Ni/bentonite oxidization and NiO/bentonite reduction kinetics, concluding to the superiority of R3 and D4 models for reduction and oxidation, respectively. The R3 model was also found satisfactory in studies by Ishida et al. [29], [30], [31], Mattisson et al. [32], [33], Moghtaderi and Song [34], Chen and Shiue [35] and Abad et al. [36] for the reduction of NiO on various supports. However, it was suggested by Adánez [2] that the R3 model is not suitable for describing the reduction of oxygen carriers of small size and high porosity. Also, Hossain et al. [37], [38], [39], [40] found that their experimental data can be better matched by the nucleation and nuclei growth model with a random nucleation mechanism (the AE1 model). The AE1 model was also selected by Son and Kim [41] to fit their NiO reduction data. Readman et al. [42] used in-situ powder XRD to study NiO supported on NiAl2O4, showing that the NiAl2O4 support remained inert during the redox process.
In summary, the lack of agreement between the various analyses is striking. The variability of the rate mechanisms adopted could be a consequence of variations in the experimental conditions or in the morphology of the Ni/NiO samples. Kruggel-Emden et al. proposed new empirical correlations for the reduction reactions of CaSO4 and Mn3O4 and oxidation reactions of Cu, Ni, and MnO, which achieved better fits compared to the common kinetic models [43]. The present work aims to analyze statistically and experimentally solid-state models that describe the reduction and oxidation of unsupported and supported NiO and Ni, which was not studied as comprehensively by Kruggel-Emden et al. [43]. In Sections ‘Solid-state kinetic model and mechanisms’ and ‘Model discrimination and identification methods’, the methodologies used for modeling and statistical discrimination are briefly discussed. Section ‘Reduction of unsupported NiO’ statistically analyzes solid-state models applied to describe H2 reduction of unsupported NiO of different origins and morphology from the literature. In Section ‘Reduction of supported NiO’, reduction experiments of supported NiO by H2 from the literature and in-house TGA tests are examined. The best solid-state reaction model is selected for each case through statistical comparison of the capability of each model to match the experimental data. The effect of temperature in the range of 600–950 °C on selected kinetic models is investigated, supported experimentally by scanning electron microscopy (SEM) and in-situ hot-stage XRD. Analysis of the oxidation kinetics of supported Ni is presented in Section ‘Oxidation of supported NiO’.
Section snippets
Solid-state kinetic model and mechanisms
Fig. 1 presents the schematic description of the models of Table 1; commonly categorized as reaction-order (F), geometrical contraction (R), diffusion (D), and nucleation (Avrami–Erofe'ev, AE). The same ordering and categorization was followed in Table 1. In summary, the reaction order-based models (F) assume a homogeneous reaction process. Mechanisms R2 and R3 of Table 1 are forms of the classic shrinking core model with phase-boundary reaction control of different geometries [44]. The R2
Model discrimination and identification methods
Common approaches of evaluating the kinetic parameters of a reaction include isoconversion and isothermal reactivity analysis methods [14]. The former encompass the stationary point method [48], Kissinger's method [49], Friedman method [50], Flynn–Wall–Ozawa method [51], [52], Kissinger's–Akahira–Sunose (KAS) method [53] and Málek's method [54]. In this study, the isothermal method was chosen because CLC is typically operated within a rather narrow temperature range [2]. Additionally, as shown
Results and discussion
This study uses 20 different solid-state reaction models (Table 1) to match experimental data obtained from the literature and this work. The models in Table 1 include 17 one-parameter models, 2 two-parameter models (RPM and AEn) and 1 three-parameter model (SB). Reduction data of 10 NiO materials (unsupported and supported) by H2 and oxidization data of 4 Ni based oxygen carriers by air are analyzed. The quality of fit (RSS) of all the models against the experimental data and their
Conclusions
The summary of the selected solid-state kinetic models for the reduction of supported and unsupported NiO by H2 and the oxidation of supported NiO by air is shown in Table 19. On the basis of the Hancock and Sharp analysis, as well as model fitting methods the two-parameter Avrami–Erofe'ev model with n ranging from 0.9 to 1.6 is shown to be the most successful in representing reduction kinetics of supported and unsupported NiO. Despite the remarkable inconsistency in previous analyses, the
Acknowledgments
This material is based upon work supported by the National Science Foundation under Grant No. 1054718. Support by W.R. Grace & Co. by providing the Al2O3/SiO2 matrices is gratefully acknowledged.
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