Fuel from Sunlight and Air - Demonstration, Automation and Parameter Analysis

Abstract

Developing novel renewable technologies for producing carbon-neutral transportation fuels has become a global energy challenge. Especially for long-haul aviation synthetic drop-in fuels are a viable option to replace refined fossil fuels. A promising pathway is the production of drop-in fuels made from CO2 and H2O using concentrated solar energy as the source of high-temperature process heat. However, so far the technological readiness level has largely been limited to bench-top studies of individual components. This thesis reports on the technological demonstration under real field conditions of the entire process chain to drop-in fuels from concentrated sunlight and ambient air. Crucial to this accomplished milestone is the design and integration of three thermochemical conversion units: A direct air capture unit for the coextraction of CO2 and H2O directly from air, a solar redox unit performing the solar redox co-splitting of CO2 and H2O to produce a desired syngas mixture, and the gas-to-liquid synthesis unit converting the syngas to liquid methanol or hydrocarbon fuels. This thesis presents the components of the implemented process chain, with a focus on the solar redox unit as the core process. It presents the fully automated full day cyclic production of syngas suitable for either methanol or Fischer-Tropsch synthesis, demonstrating the stability and robustness of the system. The demonstration of the implemented process chain is concluded with a multiple day production campaign for producing syngas that is later on transformed to methanol. A parametric study of the main operational parameters (namely: reactor pressure, reduction-end and oxidation-start temperatures, CO2 and H2O mass flow rates) determines the influence on the key performance indicators such as the specific fuel yield, molar conversion, and solar-to-fuel energy efficiency. This thesis shows how the syngas product quality can be tailored for Fischer-Tropsch synthesis by selecting adequate oxidation conditions, eliminating the need for additional downstream refining of the syngas. Changing process parameters such as reduction/oxidation temperatures, gas flow rates, or oxidation start/end conditions allows optimising the cycles towards maximising either efficiency, quality, yield or conversion. The entire solar fuel system is fully-automated based on real-time product gas analysis and feedback control loops, and can be further extended with an auto-optimisation scheme that executes online mass and energy balances to guide performance improvement. An example of a solar run of fully-automated consecutive redox cycles is presented to show the implementation of this control scheme for the optimisation of the solar fuel system. A dynamic grey box model of the redox reactor is developed for the purpose of further examining the dependence of reactor outputs to reactor inputs and investigating different operation procedures to run the reactors. The developed reactor model can also be used as building block for a future model of the entire solar fuel system. A simplified model to simulate a two reactor system and a three reactor system that make continuous use of the solar power incident on the system is applied. It allows the comparison of the two systems and helps to determine the feasibility of incorporating a third reactor of similar design into the system. The analysis shows that, for a solar power of 4.8 kW, the three reactor system is not a viable option, as additionally to the more complex implementation, it also shows 36.6% lower hourly production rate and solar-to-syngas energy efficiency. However, since, for a solar power of 6.2 kW, the two rector system is forced to experience times where both reactors are off-sun in order to keep producing syngas, introducing a third reactor when operating at higher power levels might be a viable option to keep making uninterrupted use of the incoming solar radiation.