Numerical investigation on the carbonation of CaO particles in a supercritical CO₂ fluidized bed reactor with immersed tubes

The integration of calcium-looping thermochemical energy storage system with the supercritical carbon dioxide (SCO₂) Brayton cycle offers a promising technical scheme to improve the energy conversion efficiency of the concentrated solar power plant. The carbonation reactor still faces challenges such as a serious decline in reactivity during multicycle carbonation-calcination reactions. In this study, a novel solution using sCO₂ as the fluidizing agent to improve the fluidization quality and avoid the agglomeration and sintering of CaO/CaCO₃ particles in a carbonation reactor is proposed. The effects of total gas pressure and CO₂ partial pressure on the fluidization quality, bed-to-tube heat transfer coefficient (HTC), and carbonation reaction are investigated by using the Eulerian−Eulerian two-fluid model. The results indicate that increasing the gas pressure above the CO₂ critical pressure leads to the particulate fluidization of Geldart B-type Ca-based particles. A more uniform distribution of local bed-to-tube HTC around the tube but with a reduced circumferential average HTC (by 45–119 W∙m⁻²∙K⁻¹) is achieved when using sCO₂. An increase of CO₂ partial pressure from 40 to 4000 kPa leads to a nearly 1000-fold increase in conversion rate, indicating that special designs are necessary to handle the potential over-temperature problem inside the supercritical pressure fluidized bed reactor.