Thermal reduction of iron–manganese oxide particles in a high-temperature packed-bed solar thermochemical reactor
Introduction
Thermal energy storage (TES) allows for continuous operation of concentrated solar power (CSP) plants [1], [2]. Compared to the conventional sensible and latent heat TES systems, thermochemical energy storage (TCES) is a promising alternative as it provides stable energy storage at higher density and temperature [3]. TCES is compatible with a wide variety of media, such as metal oxides, metal hydrides, carbonates, hydroxides, ammonia, and organic materials [4], [5]. TCES based on metal oxide redox cycles offers numerous advantages, including high operating temperature in a flexible range, high energy storage density, convenient operation using air as both the reactant and heat transfer medium, and simple product separation due to gas–solid reactions [6], [7], [8], [9]. In a two-step metal-oxide redox cycle, the solar energy is stored in a high-temperature endothermic reduction step and later released in an exothermic oxidation step in the form of heat,
The redox cycles of various metal oxides have been extensively studied [10], [11], [12], [13], [14]. Of particular interest is the Fe/Mn binary metal oxide system, bearing promising TCES characteristics—fast oxidation kinetics, high cycle robustness, narrow thermal hysteresis, and high energy storage density [15], [16], [17], [18], [19]. The binary oxide system with a Fe/Mn molar ratio of 2:1 (Fe67) has been developed and characterized as an attractive medium for TCES [20], [21], [22].
A laboratory-scale solar thermochemical reactor was proposed by Wang et al. for the thermal reduction of fluidized manganese oxide particles [23]. An optimized compound parabolic concentrator (CPC) is incorporated into the reactor to concentrate the incident radiation, reduce the spillage loss, and increase the uniformity of the flux distribution [24], [25]. The thermal performance of the reactor operated in the fluidized-bed mode was evaluated numerically using the Kunii–Levenspiel model [23], [26]. Despite its capability to predict the effective properties of the reactor, the model lacks the inherent physics for in-depth simulation of high-temperature solid–gas reactive flows.
High-temperature solid–gas reactive flows are frequently encountered in diverse solar thermochemical processes, such as cracking [27], [28], gasification [29], [30], [31], [32], [33], thermal reduction of metal oxides [34], [35], [36], [37], and calcium carbonate calcination [38], [39], [40], [41], [42]. Numerous models have been developed for high-temperature solid–gas reactive flows for various flow configurations: dilute particle suspensions, fluidized beds, and fixed and moving bed of porous media and packed particles [43], [44]. Typical studies on dilute suspensions focus on numerical models that couple reaction kinetics with radiative heat transfer [32], [33], [34], [35], [36], [42]. In these studies, the radiative heat transfer of the non-gray non-isothermal absorbing–emitting–scattering particle suspension under direct solar irradiation is predominantly modeled with the Monte Carlo ray-tracing (MCRT) method. Studies on fluidized beds deal with, in addition to the heat transfer and kinetics, the momentum transport introduced by the complex motion of particles. Example studies include coal gasification under direct [29], [30] and indirect irradiation [31], CO2 capture [45] and energy storage [46] based on the calcium carbonation–calcination cycle. The momentum transfer in the fluidized bed can be modeled with an Eulerian approach [47], [48] or a Lagrangian approach [49], [50], [51], [52], [53]. Fixed and moving beds of porous media and packed particles generally have higher solid-phase volume fractions, resulting in non-negligible thermal diffusion within the solid phase. Previous studies encompass modeling heat transfer in redox cycles of zinc [54], [55], [56], [57] and ceria [58], [59], [60], [61], [62], carbonaceous waste gasification [63], and CO2 capture based on the carbonation–calcination cycle [64].
In this paper, we propose a comprehensive transient three-dimensional (3D) multiphase computational fluid dynamics (CFD) model to study the reduction of indirectly irradiated packed Fe67 particles in the high-temperature solar thermochemical reactor proposed in [23]. The model couples reaction kinetics and fluid flow with the conductive, convective, and radiative heat transfer in packed Fe67 particles to obtain a detailed description of the transport phenomena in the bed. A reactor prototype that features a reaction tube confining the packed particles and a surrounding diffuse reflective cavity is tested under simulated high-flux solar irradiation provided by a multi-source high-flux solar simulator (HFSS) [65], [66]. Model validation is achieved by comparing the numerically predicted temperature profiles and oxygen generation rates with the experimental data. The validated model is then applied to evaluate the thermochemical performance of the reactor.
Section snippets
Experimental setup
The experimental setup is shown in Fig. 1. The reactor is heated by the high-flux simulated solar irradiation provided by an HFSS, which consists of 18 identical radiation modules arranged in two concentric rings [65]. The HFSS can provide 10.6 kW of radiative power with a peak flux of 9.5 MW m−2 on a 60-mm diameter flat target at its focal plane. The radiation modules of the HFSS are aligned with photogrammetry and calibrated with a flux gauge, a charge-coupled device (CCD) camera, and a
Numerical model
The numerical model is divided into three sub-models due to the complexity of the problem. We first present the hydrodynamic and heat transfer model with conduction and convection. The model assumes local nonequilibrium—each phase is described with its own energy and momentum conservation equations. The equations are coupled by closure laws for interfacial energy and momentum transfer. Secondly, a reaction kinetics model is developed for the simulation of the heterogeneous reaction. The
Physical properties
The properties of materials used in the numerical simulation are listed in Table 9. The boundary value is used when properties are required beyond the application range.
It was observed that the quality of the CPC reflective surface gradually deteriorates due to dust deposition and vapor condensation throughout the experiment. To quantify this effect, we propose an indirect experimental–numerical method to calculate the effective absorptance of the CPC from the temperature rise of the cooling
Mesh size, time step, and ray number
The structured hexahedral mesh is built for the computing domains of the reactive, participating packed particles and the confining reaction tube for accurate and stable simulation of the multiphysical process. The non-structured tetrahedral mesh is used in the rest of the domains, including the cavity and insulations. A minimum thickness of 0.21 mm is selected for the first layer of mesh on the particle–tube interface, and the irradiated tube and cavity surfaces as the highest energy flux are
Results and discussion
The validated model is applied to analyze the thermal performance of the high-temperature solar thermochemical reactor.
Conclusions
A transient 3D computational fluid dynamic model has been developed to study the reduction of iron–manganese oxide particles in a high-temperature packed-bed solar thermochemical reactor. The model coupled the reduction kinetics and the hydrodynamics with the conductive, convective, and radiative heat transfer to give a comprehensive description of the transport phenomena in the reactor. The solar thermochemical reactor featuring a reaction tube confining the packed particles and a surrounding
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank Dr Roman Bader, Dr Joe Coventry, Dr Peter Kreider, Dr Keith Lovegrove, Dr John Pye, and Dr Jose Zapata for discussing the solar reactor design. We thank Dr Mahesh Venkataraman for his help with measurements of the gas phase composition, Mr Mustafa Habib for the calibration of the ANU high-flux solar simulator, Mr Morteza Hangi and Ms Sha Li for discussing the volume-averaging theorem, and Mr Colin Carvolth, Mr Jason Chen and Mr Kevin Carvolth for the technical support. We thank Prof.
Funding
This work was supported by the Australian Renewable Energy Agency [grant number 2014/RND005].
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- 1
Current affiliation: SkyCell AG, 8005 Zurich, Switzerland.
- 2
Current affiliation: School of Science, Engineering and Information Technology, Federation University, VIC 3350, Australia.