Solids Flow in a “Particle-in-Tube” Concentrated Solar Heat Absorber

Abstract

Fluidized particle-in-tube solar absorbers are increasingly investigated both as receivers in a solar power concept or as a receiver/reactor for the thermal decomposition of minerals or for gas–solid reactions such as CO₂ looping concepts and thermo-chemical energy storage. Such applications require a high heat transfer rate from the tube wall to the upflowing suspension of particles and a strict control of the particle residence time. Conversion and heat transfer depend upon the particle mixing, its residence time (RT), and residence time distribution (RTD). Both parameters are hence important in fluidized bed applications. The present research experimentally investigated the use of an Upflow Bubbling Fluidized Bed (UBFB) as a particle-in-tube concentrated solar receiver and/or reactor. The RTD was determined by tracer response and compared with predictions from different models. Both a cascade of stirred tank reactors and a plug flow with dispersion model provide a good fitting of the experimental results. The former is however preferred, with a number of cascade mixing cells between 3 and 4, slightly dependent on both the operating air velocity and the particle circulation rate. The commonly applied ratio of the superficial fluidization velocity and the minimum fluidization velocity of the particles is between 3 and 20, whereas solid circulation fluxes up to 100 kg/m² s are used. Design equations are derived. The results are moreover used in test cases of a concentrated solar power absorber and in the use of a UBFB as solar limestone calcination furnace. The approach can also be applied to processes of, for example, heat absorption or drying provided kinetic data are known.