(135e) A Numerical and Experimental Investigation of a High Temperature Fluidized Bed for Thermal Energy Transfer

Several new thermal energy storage systems use silica sand as the storage medium and heat transfer fluid because sand is relatively inexpensive and stable at very high temperatures. In the energy system we investigate, surplus electricity from the energy grid is used to heat particles to 1200 °C. When required, the heated particles are fed to a pressurized fluidized bed heat exchanger, where the hot fluidization gases are then used to drive a turbine. A laboratory-scale pressurized fluidized bed (PFB) and discrete element model (DEM) are built to study the heat transfer and mixing phenomena in the PFB. NETL’s open-source code, MFiX, is used as the computational tool to model the PFB. The MFiX-DEM is built to match the experimental setup with some reductions to improve the computational efficiency. The domain is a thin 3D rectangle with the height and length matching the experimental setup and a depth corresponding to 20 particle diameters. The PFB is run at 1100 °C and 10 bar pressure with the fluidizing air entering at 200 °C. The fluidization behavior, mixing, and particle heat transfer measurements are compared to the DEM model and existing empirical correlations. A good agreement of within 10% between the model and experimental is obtained for the bed pressure drop, defluidization curve, and particle heat transfer. In the experimental setup, we measure an initial temperature gradient within the bed. The temperature gradient arises due to higher temperature air on top of the bed relative to the bottom of the bed near the distributor. To approximately model the non-uniform initial bed temperature, the bed in the DEM simulation is initialized as having 3 sections of different temperatures. At superficial velocities very close to the minimum fluidization velocities, we observe that the bed is partially fluidized i.e., the hot regions are fluidized and bubbling while the colder region remains unfluidized. This phenomenon is observed experimentally as well. It is also observed that cold particles tend to settle at the bottom of the bed, regardless of where they are at the beginning of the fluidization process. This has a significant effect on the bed mixing and the overall pressure drop in the bed and hence affects the thermal storage and heat transfer properties.