Thermal Behaviour of Compartmented Fluidized Beds Under Uneven Fluidization Conditions

Particle solar receivers in Concentrated Solar Power (CSP) applications are becoming more and more relevant to develop technologies able to be coupled with high-efficiency thermodynamic cycles for energy production and/or with systems of thermal and thermochemical energy storage. As matter of fact, granular materials can be, in principle, operated as heat transfer fluid (HTF) and as energy storage media at high temperature (>1000°C) overcoming the technological issues related to the adoption of other HTF like molten salts. Among particle receivers, dense gas-solid fluidized suspensions have been recently proposed thanks to their excellent thermal properties, namely bed-to-wall heat transfer coefficient (several hundreds of W/m²K) and effective thermal diffusivities (0.001-0.1m²/s) associated with convective transfer due to bubble-induced and gross bed solids circulation. Both these features may be optimized by proper selection of fluidized solids type and size and fluidization regime. Non-conventional design and operation of fluidized beds based on uneven or unsteady (pulsed) fluidization may further enhance their thermal performances for CSP and thermal energy storage applications. Dense gas-solid fluidized beds have the potential to effectively accomplish three basic complementary tasks: a) collection of incident solar radiation; b) transfer of the incident power to heat exchange surfaces and henceforth to high-efficiency power cycles; c) thermal energy storage, aimed at equalizing the inherent time-variability of the incident radiation for stationary combined heat and power (CHP) generation. All these features have been exploited in a novel concept of solar receiver for CHP generation with inherent thermal energy storage. The concept is based on a novel design of the solar collector, based on a compartmented dense gas fluidized bed optimized so as to accomplish the three complementary tasks. The hydrodynamics of a near-2D dense gas-fluidized bed operated at ambient conditions and equipped with a compartmented distributor of fluidizing gas, consisting of two spargers, has been characterized to accomplish the effectiveness of compartmented fluidization of the bed without physical separations or internals inside the bed. Uneven fluidization was accomplished by partitioning of the fluidizing gas between the sections of the distributor: a larger gas superficial velocity (exceeding the incipient fluidization velocity) was established in the “active” section of the distributor, a smaller (or even 0) gas superficial velocity was established in the passive section of the distributor. Results confirm the expected inherent tendency of the bed to equalize uneven fluidization. Crossflow of fluidizing gas from the active to the passive sections of the bed, driven by lateral pressure gradients, promotes fluidization of the passive section and/or defluidization of the active section of the bed. Crossflow becomes increasingly important as the bed depth increases and this makes compartmented fluidization of deep beds more problematic. However, it was proven that proper fluidization maps can provide the basis for the design and operation of gas distributors to effectively achieve compartmented fluidization of the fluidized bed.

This work aims at investigating the thermal behaviour of compartmented fluidized beds under even and uneven fluidization conditions. In particular, the transversal thermal diffusivity was studied both by experiments carried out in the compartmented near-2D dense gas-fluidized bed and by CFD numerical simulations.

The experimental apparatus consists of a near-2D fluidized bed (2850x1860x200mm) equipped with an array of pressure and temperature taps at different locations in the bed. The fluidized bed can be considered as “nearly two-dimensional” because it is characterized by a thickness much smaller than the other dimensions, but at the same time large enough to prevent extensive wall effects for bubbles smaller than 120 mm. Accordingly, the test facility can be used to investigate bed hydrodynamical patterns along the width and the height of the bed as they would develop in full-scale 3D compartmented fluidized beds. The fluidization column is equipped with different diagnostic taps and inlet and outlet ports, necessary for air distribution system, temperature monitoring system, heating system and for discharge of bed material. The bed was equipped with two spargers acting as gas distributors, intentionally of different length so that it was possible to establish uneven fluidization of the bed with asymmetric patterns. Accordingly, the spargers, and the corresponding sections of the fluidized bed are called long and short. The bed material was fine silica sand with a mean Sauter diameter of 145 µm. Fine bed solids were chosen for CSP applications in order to obtain large heat transfer coefficient and effective thermal diffusivity even at small fluidization velocities. A temperature measurement system based on thermocouples was used to map thermal conditions inside the fluidized bed. The system consists of three movable probes which can be immersed directly in the bulk of the bed at different levels above the distributor. Additionally, four fixed probes are located at different locations at the lateral walls. The three movable probes are vertically inserted inside the bed from the top side of the apparatus. Two heating cartridges are located inside the short compartment and are regulated to keep the short compartment at a constant temperature. At steady state conditions the thermal behaviour of the fluidized bed under even and uneven fluidization conditions was evaluated by mapping the bed temperature at different vertical and horizontal positions. A simple model of the experiment was developed to estimate the transversal thermal diffusivity as a function of operating conditions investigated.

The experimental conditions investigated was also studied by CFD simulations using a Eulerian-Eulerian modelling approach. The main aim of the numerical simulations was to reproduce the experimental results to consolidate the proper choice of the frictional solid stress to properly capture the hydrodynamics of compartmented fluidized beds. In particular, the mixing length scales of the solid phase responsible of transversal thermal diffusivity under even and uneven fluidization conditions were specifically investigated to individuate the establishment of gross bed solids circulation.