Padula, Stefano (2022) Directly Irradiated Fluidized Bed Autothermal Reactor for solar thermochemical processes and energy storage. [Tesi di dottorato]

Anteprima

Testo PADULA_STEFANO_XXXV.pdf Download (5MB) | Anteprima

Abstract

This Ph.D. thesis relates the development of a new solar reactor, named Directly Irradiated Fluidized Bed Autothermal Reactor (DIFBAR), that exploits fluidization technology and the principle of an autothermal reactor to carry out solar chemical processes with high efficiency. The viability of recovering the sensible energy of the solid products to preheat the reactants is studied in a continuous solar reactor with an innovative design coupling a cavity receiver and a solid-solid countercurrent heat exchanger, made by two coaxial tubes. This objective has been pursued through modeling and experimental activities, to design, build and test a fully operational prototype. A simple compartmental model to understand the effect of the design and operational variables and to assess the application of the DIFBAR for ThermoChemical Energy Storage (TCES), taking the Calcium Looping (CaL) as a reference process has been developed. An optimal temperature receiver temperature was found between 890-900°C. The heat exchanger was chosen to be 1 m long and to operate with a solid mass flowrate of about 1.4 g/s. A parallel experimental study has been carried out on a solar process for the production of hydrogen through redox cycles. In particular, a laboratory-prepared perovskite with chemical formula La0.6Sr0.4FeO3 has been tested for Chemical Looping Reforming of methane (CH4) with bench reactors, in view of an application with the new DIFBAR prototype. The time-resolved concentration profiles of the outlet gas have been analyzed to assess reaction temperatures, oxygen capacity and selectivity. The material stability has been tested by iterated cycles. The effect of gas-solid contact patterns has been scrutinized by comparing the performance of fixed and fluidized bed reactors in terms of conversion rates and selectivity. The results encouraged to test the material with a directly irradiated fluidized bed reactor, reproducing the geometry of the DIFBAR receiver. A batch of perovskite was mixed with the reactor inventory, made of mullite particles. The conversion rates did not match those under fluidized bed conditions, maybe as a result of a physical-chemical interaction between the perovskite oxygen carrier and the mullite. The new DIFBAR prototype features a closed circulation loop of the solid composed by a fluidized bed riser, a solid separator (the receiver), a standpipe (the annulus) and a reservoir. Cold flow experiments have been carried out with Geldart B sand to verify proper control of the system, as a preliminary step toward high temperature experiments. The solid circulation rate can be regulated through the riser fluidization velocity and match the design target of 1.4 g/s. Pressure measurements have been used to monitor and control the bed level in the annulus. Internal gas flow patterns have been determined by a gas tracing technique, indicating that undesired gas by-passing streams are very small and can be zeroed by regulating the operating conditions. An in-house built high-flux solar simulator has been used for high temperature experiments. A characterization of the heat exchanger was carried out by operating the prototype with the same inert sand. The results have been used to validate the compartmental model, extended to simulate transient operation. The heat transfer coefficient ranges between 370 and 540 W/(m2 K) and a heat recovery factor of 90% has been calculated. The receiver temperature reached 700°C and was sufficiently high to perform calcination tests with MgCO3, providing a first demonstration of the working principle of the DIFBAR under reactive conditions. The work done lays the groundwork for future studies aiming at the optimization of the reactor and testing new solar processes.

Downloads

Actions (login required)

Modifica documento