CFD-DEM Simulation of a Moving Packed Bed Direct Contact Heat Exchanger for Solar Fuel Production

Kurzfassung

Green hydrogen and synthetic fuels can be produced with thermochemical two-step cycles powered by concentrated solar radiation, for which several reactor designs exist. Specifically, a concept based on moving reactive ceria particles and nitrogen as sweep gas has been developed by the authors in the past years. Its core component is a moving bed heat exchanger and reactor, where hot particles at 1750 K enter at the top, and a sweep gas at colder temperatures of approximately 1100 K enters at the bottom. Sweep gas and particles move counter-current to each other and are in direct contact. This not only cools down the particles, but also reduces them under the release of oxygen. The heat exchanger effectiveness of such a component is theoretically predicted as high as 99 %, but literature values of real systems are substantially lower (50 %−96 %). To assess the performance in more detail, a validated CFD-DEM simulation of the system is conducted. The simulation has been verified and validated by comparison to analytical solutions, empirical correlations and a transient fixed bed experiment. In addition, it has been confirmed that the energy balance in the system is met. The CFD-DEM simulations reveal heat exchanger effectiveness values between 73 % and 94 % for first prototype designs. The parametric study shows that the actual value strongly depends on the geometry of the moving bed heat exchangers