Heat and mass transfer model of a packed-bed reactor for solar thermochemical CO₂ capture

A 1 kW_th dual-cavity solar thermochemical reactor concept is proposed to capture carbon dioxide via the calcium oxide based calcination-carbonation cycle. The reactor is oriented beam-up wherein concentrated solar energy from a heliostat field enters an aperture located at the bottom of the reactor. In the endothermic calcination step, concentrated solar radiation is captured by the inner cavity and transferred by conduction through a diathermal cavity wall to the particulate CaCO₃ medium located in the annular reaction zone. The liberated CO₂ is removed from the reactor to external storage. In the exothermic carbonation step, a CO₂-containing gas flows through a bed of CaO particles in the reaction zone, forming CaCO₃, while CO₂-depleted gas leaves the system. The reactor design is refined using a numerical heat and fluid flow model for the calcination step. The Monte Carlo ray-tracing method is employed to solve for radiative exchange in the inner cavity, coupled with a computational fluid dynamics analysis to solve the mass, momentum, and energy equations in the concentric reaction zone modeled as a gas-saturated porous medium consisting of optically large semitransparent particles. The cavity diameter and length-to-diameter ratio are varied to study their effects on pressure drop, temperature distribution, and heat transfer in the reactor. Increasing the cavity diameter and length-to-diameter ratio decreases the radial temperature gradients across the cavity wall and within the reaction zone. However, it also results in increased pressure drop and reduced heat transfer to the reaction zone.