Directly Irradiated Fluidized Bed Autothermal Reactor (DIFBAR): High-Temperature Operation

There is growing interest toward the application of Concentrating Solar Thermal (CST) technologies to endothermic processes, involving gas-solid chemical reactions. The purpose of this work is to advance the concept of the Directly Irradiated Fluidized Bed Autothermal Reactor (DIFBAR), a newly-designed solar reactor, that exploits fluidization technology and the principle of an autothermal reactor: the thermal energy of the solids flowing out of a solar receiver is used to preheat the feed in a countercurrent double-pipe heat exchanger. This allows to reach high temperatures with minimum solar power input, reducing the size of the CST facility. A full DIFBAR prototype has been tested with a high-flux solar simulator, consisting of an elliptical reflector and a 10 kW_el Xe arc lamp. The prototype is operated as a circulating fluidized bed: the inner tube of the heat exchanger is a fluidized bed riser, the receiver works as a gas-solid separator, the outer tube (annulus) of the heat exchanger is an overflow standpipe and a buffer tank (reservoir) connects the annulus to the riser, closing the loop. The reservoir can also be operated as a fluidized bed reactor like in a dual-fluidized bed system. Experiments confirm the viability of the autothermal operation. Results, obtained for different bed materials (Geldart B class), cover all the fundamental aspects of the system: hydrodynamic control, thermal behavior and chemical reaction. Solids circulation rates up to 9 kg/h are measured at ambient temperature, while varying the gas velocity in the riser. Heating tests are carried out for different irradiation powers. Steady state temperature profiles are analyzed to assess the performance of the heat exchanger: the heat transfer coefficient is evaluated between 340 and 490 W/(m² K). Preliminary tests involving the calcination of magnesium and calcium carbonate are carried out to investigate the operability of Chemical Looping processes.