CFD simulation investigation of size-induced thermophysical characteristics of biomass materials in a fast pyrolysis reactor

Biomass pyrolysis is a crucial method for improving energy efficiency. However, the effects of segregation on the thermochemical properties of biomass pyrolysis in bubbling fluidized beds (BFB) are still not well understood. This study investigates biomass pyrolysis in a lab-scale BFB using the MP-PIC model, considering both heterogeneous and homogeneous reactions. After validating the model with experimental data, the size- and reaction-induced segregation and thermochemical properties were analyzed. The results show that axial segregation due to reaction and particle size differences leads to a continuous decrease in sand particle size along the bed height, significantly affecting solid heat and mass transfer. The heat transfer coefficient (HTC) of biomass and sand particles peaks near the biomass inlet but decreases for biomass as it moves upward. Increasing superficial velocity enhances biomass pyrolysis and improves the uniformity of temperature and heat carrier quality. Additionally, raising the bed temperature increases both biomass temperature and HTC. For sand particles of varying sizes, larger particles show higher temperatures, lower HTC, greater slip velocity, and reduced dispersion, while smaller particles exhibit the opposite. These findings provide valuable insights into segregation and thermochemical behavior in BFBs during high-temperature pyrolysis.