Modular reactor model for the solar thermochemical production of syngas incorporating counter-flow solid heat exchange

Recent progress in thermochemical reactor concepts, among them batch, particle and counter-rotating configurations, shows different approaches to heat recuperation and gas separation. An idealized physical analysis in search of best-efficiency potentials is required, that covers multiple degrees of freedom in configuration space from batch to quasi-continuous concepts. A modular and generic solar thermochemical reactor model is thus presented that describes two-step redox reactions of solid pieces of reactant moving in counter flow between reduction and oxidation chambers to produce CO and H-2 from CO2 and H2O. Solid heat recuperation is implemented through radiation heat exchange between reduced and oxidized elements moving in opposite directions. The model can be adapted to a wide range of reactor concepts and is validated through a comparison of results with models presented in the recent literature. The model is demonstrated in an upper-bound performance analysis with an example of a ceria system, where a heat exchanger efficiency of about 80% is reached. Using the minimum thermodynamic energy for the vacuum pump and gas separation, the physical potential of cycle efficiency at the modeled temperatures at a reduction oxygen partial pressure of 10(-3) atm is 22% and may be increased to 33% at a partial pressure of 10(-5) atm. With assumptions on practical energy requirements the efficiency is reduced to 16% at 10(-3) atm. In order to quantify the dependence of the heat exchanger efficiency on internal heat transfer characteristics, heat propagation by radiation and conduction through the reactive material and insulation is modeled in an example case of a porous medium. This model provides the basis for future analyses of different reactor concepts including parameter studies of the number of heat exchanger chambers, the residence time and reaction temperatures. (C) 2015 Elsevier Ltd. All rights reserved.