Solar Fuel Production by Thermochemical Particle Processes

Abstract

The use of thermochemical cycles for splitting water with the help of concentrated solar power provides an alternative and sustainable route for hydrogen production. Together with the ability to simultaneously split CO2 into carbon monoxide, these processes enable the production of synthesis gas, which is the building block for synthetic fuels and also widely used in the chemical industry. The splitting in a thermochemical two-step cycles uses a redox material to divide the process in a reduction and an oxidation step. During the reduction step (1), the redox material is regenerated by using high temperatures up to 1500 °C to release oxygen from its lattice structure. By exposing the activated material to water vapor and/or carbon dioxide at temperatures around 800-1000°C, oxygen ions are split off the gases and the material is re-oxidized, leaving hydrogen and/or carbon monoxide as product (2). M_ox -> M_red+O_2 (1) M_red+H_2 O/CO_2 -> M_ox+H_2/CO (2) Several processes using this concept were developed in the past, using mainly locally fixed monolithic structures of the redox materials. Their operation could show the general feasibility but suffered from inefficiency because of heat losses because of necessary temperature swings. This work will present alternative concepts with solid, moving particles of the redox material, which overcome aforementioned restrictions and allow better scalability. The solid particles are moved in a closed cycle. The reactions are separated spatially in different components, which allow for continuous operation. Particle/gas heat exchangers allow for heat recovery and modelling results suggest solar-to-fuel efficiencies up to 20%. Test settings and results of reactors operating with particles under concentrated irradiation will be presented. DEM/CFD simulations were used for design optimization of particle transport and flow. Furthermore, the particle selection and their properties will be addressed, as they influence the flow behavior and affect component design. Finally, current challenges regarding the material, design and operation will be discussed.