A novel autothermal fluidized bed reactor for concentrated solar thermal applications

In the present study a novel directly-irradiated fluidized bed autothermal reactor (DIFBAR) for solar-driven chemical processing is presented. The reactor exploits the very concept of the autothermal reactor applied to multiphase flow of fluidized solids: a gas-solid suspension is preheated in a tubular riser prior to being exposed to radiative flux and further heated up in a concentrated solar receiver. Preheating is accomplished by solids leaving the receiver as they flow countercurrently in an annular dipleg coaxial to the riser. The dipleg may be designed so as to make the receiver section gas leak-tight with respect to the other zones of the reactor. An experimental campaign was performed under non-reactive conditions and at moderate radiative flux, hence moderate temperatures, with the purpose of demonstrating the feasibility and operability of the autothermal reactor, as a preliminary step toward a full proof-of-concept. Solids circulation rates and temperatures were measured as a function of the gas superficial velocity in the riser under given simulated solar irradiance. Infrared thermography was applied to further characterize multiphase flow and thermal patterns in the receiver. Experimental data were worked out to calculate the annulus-to-riser heat transfer coefficients. The reactor demonstrated excellent "autothermal" operation, thanks to very efficient heat transfer across the riser-annulus interface. Collection of solar energy was fairly good, with maximum thermal efficiency close to 70%, despite the design of the receiver was not specifically optimized to minimize radiative and convection losses. A preliminary analysis aimed at the scale-up of the autothermal reactor is presented. Altogether, results obtained are quite promising and stimulate further research on the development of reactors based on the proposed design for solar chemistry applications.