Heat transfer and chemical kinetics analysis of a novel solar reactor for hydrothermal processing

Abstract

This work proposes a novel concept of a hydrothermal solar reactor to convert biomass waste efficiently into bio-products. The operation of this system is studied using heat transfer and kinetic models to assess the concept’s capabilities. The reactor consists of a cylindrical cavity with a window that allows concentrated solar radiation to pass. Inside the cavity, radiative energy is absorbed by an array of tubes, where the reacting water biomass suspension circulates. The cavity walls are assumed to be highly reflecting, and the tube walls to be highly absorbing. Highly-scattering inert fluidized particles are utilized in the cavity to redistribute radiation prior to absorption by the tubes. The cavity-type solar reactor design aims at homogenizing the distribution of radiation absorbed by the tubes, that is, to minimize axial and angular thermal gradients and reduce thermal stresses, given the high operating pressures required for hydrothermal processes. The Monte Carlo Method (MCM) for thermal radiative transfer is used to couple the solar radiation absorption to conduction and convection heat transfer phenomena. The effect of design parameters, like the number of tubes, their location, the relative size of the cavity, and the scattering particles concentration, is evaluated in the reactor thermal performance. The results indicate that adding the participating medium and carefully choosing the number and distribution of tubes allows reactor thermal efficiencies above 80% while reducing thermal gradients within the tubes. Moreover, the analysis shows the solar reactor concept can attain biogas and biocrude yields slightly above 20% and 30%, respectively.