Numerical Investigation of a Metal-oxide Reduction Reactor for ... - Core

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 61 (2014) 2054 – 2057

The 6th International Conference on Applied Energy – ICAE2014

Numerical investigation of a metal-oxide reduction reactor for thermochemical energy storage and solar fuel production Jayaveera P. Muthusamya, Nicolas Calveta, Tariq Shamima* a

Institute Center for Energy (iEnergy), Department of Mechanical and Materials Engineering Masdar Institute of Science and Technology, Masdar City, P.O. Box 54224, Abu Dhabi, UAE

Abstract The optimum design of a thermochemical reactor involves complex phenomena such as radiation heat and mass transfer, reaction kinetics, and fluid-solid interactions which makes the process to be cumbersome. The computational modelling using computational fluid dynamics (CFD) is an important tool to study, analyse, interpret and optimize such a complex system. The present study has developed a CFD model using Ansys-Fluent 14.0. The model employs the Eulerian-Lagrangian approach with the chemical reactions for a vertical axis thermochemical reactor for Zinc oxide reduction process. The conversion efficiency and temperature distributions for Zinc oxide reduction estimated by the CFD results have been validated with the published results by the PROMES CNRS laboratory (France) for a horizontal reactor design. The effect of particle size and gravity with a view to adapt it for a vertical beam-down solar concentrator have been analysed in the present study. It was observed that smaller particles provide higher surface area for enhanced heat transfer. Gravitational effects turned to be significant as the particle size increases. The model will assist in the effort of developing an efficient solar chemical reactor, for innovative thermochemical energy storage systems or fuel production, which will be tested at the Masdar Institute‘s beam down research facility. © 2014 2014The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the Organizing Committee of ICAE2014 Keywords: Thermochemical energy storage; metal oxide; CFD;CSP; beam down concentrator; reacting multiphase flow

1. Introduction Advancements in concentrated solar power (CSP) technologies need high temperatures and high efficient energy storage and recovery methods. The efficient transformation of solar energy to chemical energy is a long term research worldwide. However, recently commercially emerged CSP technologies make thermochemical (TC) energy storage (TCES) as more attractive and promising as CSP can reach wide range of temperatures (200 °C to 3000 °C) and TCES has a number of advantages such as loss free, long term storage, and high storage density compared to conventional sensible or even latent heat thermal energy

* Corresponding author. Tel.: +971-2-810-9158; fax: +971-2-810-9901. E-mail address: [email protected]

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of ICAE2014 doi:10.1016/j.egypro.2014.12.074

Jayaveera P. Muthusamy et al. / Energy Procedia 61 (2014) 2054 – 2057

storage methods. TC system combined with CSP can also be used to produce hydrogen (H2) from water splitting cycles that is a sustainable alternative to fossil fuel. Among various TCES, metal oxide cycles using reduction-oxidation (redox) reactions have a strong potential to meet future operating temperatures and electricity cost requirements. In the first step, an endothermic reduction reaction must be carried out in a high temperature solar reactor driven by CSP, followed by an oxidation or hydrolysis step based on TCES or H2 production, respectively. As the heat recovery efficiency highly depends on the efficiency of the reduction step, a significant effort must be dedicated to its design and optimization. Some of earlier studies dealt with solar reactor modelling [1, 2, 3]. In order to avoid particle deposition and to obtain uniform heating of particles, rotating cavity receives are generally employed [3, 4, 5]. This study aims at simulating and validating a small-scale horizontal axis solar chemical reactor in line with Abanades et al., [3] and to adapt this design for vertical axis reactor with a view of in house beam down research facility as shown in Figs. 1 and 2(a). The solar reduction of zinc oxide (ZnO(s) ė Zn(g) + 0.5O2) has been simulated using CFD tool Ansys-Fluent using chemical kinetics data for ZnO dissociation from literature [6,7]. The injected ZnO particles are reduced by direct radiation, cavity wall radiation and other modes. However, in this study, experimentally measured heat flux profile on the inner cavity wall [3] as shown in Fig. 2(b) has been used.

Fig. 2. (a) Masdar Institute’s BDOE research facility

Fig. 1. Schematics of vertical axis reactor

Fig. 2.(b) Radiation heat flux profile [3]

2. Problem Formulation A 2D axis-symmetric model was created and analyzed. The model includes carrier gas flow (Argon), heat and mass transfer, chemical reaction, multiphase flow, and coupling of these phenomena. The details of the governing equations may be found elsewhere [3]. The reactive particle-laden flow was simulated by a Lagrangian discrete phase model combined with the Eulerian continuum approach solved by NavierStokes equations. The radiation effects were included using discrete ordinates (DO) radiation model. The reaction rate is determined by inherent kinetics of ZnO dissociation on the basis of the Arrhenius law [7]. Contact resistances between different solid components have been considered to obtain more realistic heat transfer and temperature distributions across the solid boundaries.

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3. Results and Discussion The numerical results were validated by comparing them with the work of PROMES [3] for different particle sizes. Figs. 3(a) and (b) shows the validation results for the conversion efficiency of 100% (all injected particles are reduced) and particle size of 0.1 micron. The results for both the temperature distribution and particle reduction are in good agreement. PROMES CNRS

Present study

Fig. 3. Validation results (a) Temperature distribution (b) ZnO reduction due to surface reaction

Fig. 4. Effect of particle size (horizontal & vertical)

Fig. 5. Temperature of cavity (different planes)

Further, it was attempted to study a vertical axis reactor for beam down CSP configuration in order to obtain the effect of initial particle diameter. Fig. 4 shows the comparison of vertical design with horizontal design. It can be observed that as particle size increases, conversion efficiency decreases for both designs. The higher the particle diameter, the lower the surface area for the reaction per mass flow rate, and thus the lower the conversion rate. Smaller particle sizes have less impact on conversion efficiency compared to horizontal axis design due to negligible gravity and drag effects. The particle size of 0.1 micron produces 100% conversion efficiency, whereas 10 micron particles produce close to 0% efficiency, which indicates the advantage of fine particles. Fig. 5 shows the temperature distribution of inner cavity wall and various planes created in the interval of 1mm from the cavity wall. It can be seen that the maximum temperature occurs at the cavity wall and gradually decreases towards the cavity region. Figs. 6 and 7 show the particle temperature remains lower than the wall temperature because a portion of energy is lost by particles due to the endothermic reaction and heat transfer to the surrounding gas. For the inlet velocity of 0.1 m/s and secondary gas velocity of 0.01 m/s, the temperature contours of vertical reactor are shown in Fig. 6. The decomposition of particles for 0.1micron particles which gives about 100% conversion efficiency is shown in Fig. 7. This result shows the importance of initial particle diameter. The subsequent particle diameter for oxidation/hydrolysis or regeneration of metal oxide for completing the cycle depends on the quenching process to recover solid metals.


Fig. 6. Temperature contours (K)

Fig. 7. Particle temperature contours (K)

4. Conclusions A vertical axis thermochemical reactor for ZnO oxide reduction was simulated using CFD analysis after validating the results with the work of [3] for horizontal design within 10 to 15 % deviation. The effect of initial particle size on the conversion efficiency has been established and compared with horizontal design. For both horizontal and vertical design, the particle size of 0.1 micron is preferred for maximum conversion efficiency. Larger particle size (above 10 micron) yields almost zero reduction, and should be avoided. Bigger particles leads to less conversion for vertical design compared to horizontal reactor due to gravity and air-resistance (drag) effects. The accomplished simulation provided the trends about the thermal performance of the vertical axis reactor and the reactivity of particles. However, other parameters like geometry, rotational speed, direct radiation, particle feed rate and quartz window can also influence the conversion process which is the scope of our future work. This methodology can effectively be used for design, development and optimization of TC reactor concepts for metal oxide reduction. Acknowledgement This research was supported by the Government of Abu Dhabi to help fulfil the vision of the late President Sheikh Zayed Bin Sultan Al Nayhan for sustainable development and empowerment of the UAE and humankind. References [1]

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