(281c) Automation and Parametric Study of a Solar Fuel System for the Thermochemical Production of Syngas from H2o and CO2

We report on a parametric study of a modular and fully-automated solar fuel system for the solar thermochemical production of syngas – a specific mixture of CO and H₂ − from CO₂ and H₂O via a ceria-based redox cycle using concentrated solar energy. The quality of the solar syngas, given by its composition, is suitable for the targeted downstream gas-to-liquid synthesis of drop-in fuels, e.g. Fischer-Tropsch or methanol synthesis.

This system is driven by concentrated solar process heat to effect the simultaneous co-splitting of CO₂ and H₂O via a ceria-based thermochemical redox cycle. The solar reactor for effecting this redox cycle consists of a cavity-receiver containing a reticulated porous ceramic (RPC) foam structure made of pure CeO₂. Two identical solar reactors are mounted side-by-side on the focus of a solar dish concentrator that enables the operation of both solar reactors simultaneously by alternating the solar radiative input between them. Thus, while one solar reactor is performing the endothermic reduction step on sun, the second solar reactor is performing the exothermic oxidation step off sun.

The characteristic redox cycle is operated under a temperature/pressure-swing mode, consisting of three phases: 1) The reduction phase, during which the the solar reactor is heated with concentrated sunlight up to the desired reduction end temperature T_{reduction,end} of up to 1500°C to release O₂ from CeO₂, assisted through lowered total pressure by a vacuum pump and an Ar sweep gas flow. 2) A cool-down phase under atmospheric pressure during which the solar reactor, re-pressurized by injecting CO₂, cools down to the oxidation start temperature T_{oxidation, start}. 3) The oxidation phase, during which CO₂ and H₂O are co-injected into the reactor’s cavity, react with the reduced ceria to form syngas.

The composition of the syngas, and in particular the H₂:CO and H2:CO_x molar ratios, is crucial for the downstream gas-to-liquid processes. Tailoring the syngas composition by changing CO₂ and H₂O feed flows as well as choosing adequate oxidation start/end conditions eliminates the need for additional downstream refining of the syngas, e.g. via the energy-intensive reverse water-gas shift reaction. The main operational parameters are: reduction pressure, Ar sweep gas flow, T_{reduction,end}, T_{oxidation, start}, and CO₂ and H₂O mass flow rates. A parametric study targeting different syngas compositions is performed to analyse the influence of these main operational parameters on the four key performance indicators, namely the syngas quality, fuel yield, CO₂ mass conversion, and solar-to-fuel energy efficiency.

The entire system is controlled to perform fully-automated consecutive redox cycles based on real-time product gas analysis and feedback control loops. To optimise the aforementioned key performance indicators, an automated control program was developed to perform mass and energy balances online, determine the improvement options based on the parametric study, and update the aforementioned main operational parameters, leading to optimized conditions. We present representative on-sun runs of fully-automated consecutive redox cycles where this control scheme is implemented for the optimisation of the solar fuel system.