Modeling counter-flow particle heat exchangers for two-step solar thermochemical syngas production

Particle reactor concepts have been suggested for the implementation of two-step solar thermochemical redox cycles due to several inherent advantages such as new heat exchanger concepts, flexibility of the reactor design, fast reaction kinetics due to high surface area of the reactive medium, and resistivity to thermal shocks. Further, heat recuperation from the solid phase has been shown to be crucial for the achievement of high efficiencies using reactive material undergoing nonstoichiometric redox reactions at different temperature levels. It is therefore of interest to investigate the potential for heat recuperation from the solid phase of the reactive material for reactor concepts based on particles. We present a model of a generic double-walled heat exchanger for the counter-flow of reduced and oxidized particles, where heat is transferred from the hot to the cold particles through a separating wall, which prevents mixing of the atmospheres. The upper and lower bound for the performance of the heat exchanger of perfectly mixed and unmixed beds are evaluated. Heat transfer in the particle beds and between the beds and the separating wall is described with published models, and the results are compared with experimental data from the literature. A parameter study is performed on a chosen system implementation with a size of recent laboratory demonstrations, where entry temperatures, geometries and residence times of the particles in the heat exchanger are varied. It is found that heat exchanger effectiveness is maximized for higher entry temperatures and for optimum values of the geometry and residence time balancing enhanced heat transfer between the particle beds and increased losses to the environment. Heat exchanger effectiveness reaches values of over 50% for unmixed beds and over 80% for perfectly mixed beds for an optimal choice of parameters. Effectiveness can be significantly enhanced through radial mixing of the beds, as the main limitation of heat transfer through the beds is reduced. Heat exchanger concepts based on the counter-flow of solid reactive particles are thus shown to have a high potential and the presented computational model is a valuable tool for the evaluation of their performance and guidance for their design. (C) 2017 The Authors. Published by Elsevier Ltd.