Heat transfer and energy storage characteristics of calcium carbonate particles in a countercurrent fluidized bed reactor

The concept of countercurrent fluidized beds (CCFB) has recently emerged as a promising design for thermochemical energy storage reactors. However, the interplays among flow structures, heat transfer characteristics, and chemical reactions within CCFBs remain unclear. This study employs computational fluid dynamics-discrete element simulations to investigate the impact of flow structures on the heat transfer and chemical reaction behaviors of calcium carbonate particles in CCFBs. The findings reveal that both the heat transfer coefficient and reaction extent exhibit an initial increase followed by a decrease as the inlet superficial gas velocity increases, highlighting an optimal operating condition at approximately 0.74 times the minimum fluidization velocity. This optimal velocity arises from the early onset of fluidization in CCFBs triggered by CO2 release and thermal expansion. Excessive local gas velocities lead to the formation of large bubbles or slugs, increasing contact thermal resistance and reducing reaction efficiency. A fluidization phase diagram is proposed to elucidate the transitions in flow state resulting from variations in local gas velocity due to CO2 release and thermal effects. Additionally, a quadratic function is proposed to predict local heat transfer coefficients based on the local gas volume fractions. These results provide valuable insights for optimizing CCFB reactor design and selecting ideal operational parameters.